what is Microsoft Windows ?

Microsoft Windows is a series of software operating systems and graphical user interfaces produced by Microsoft. Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces (GUIs). Microsoft Windows came to dominate the world's personal computer market, overtaking Mac OS, which had been introduced previously. As of October 2009, Windows had approximately 91% of the market share of the client operating systems for usage on the Internet. The most recent client version of Windows is Windows 7; the most recent server version is Windows Server 2008 R2; the most recent mobile device version is Windows Mobile 6.5.



Versions
The term Windows collectively describes any or all of several generations of Microsoft operating system products. These products are generally categorized as follows:
Early versions

The history of Windows dates back to September 1981, when the project named "Interface Manager" was started. It was announced in November 1983 (after the Apple Lisa, but before the Macintosh) under the name "Windows", but Windows 1.0 was not released until November 1985. The shell of Windows 1.0 was a program known as the MS-DOS Executive. Other supplied programs were Calculator, Calendar, Cardfile, Clipboard viewer, Clock, Control Panel, Notepad, Paint, Reversi, Terminal, and Write. Windows 1.0 did not allow overlapping windows, due to Apple Computer owning this feature[citation needed]. Instead all windows were tiled. Only dialog boxes could appear over other windows.
Windows 2.0 was released in October 1987 and featured several improvements to the user interface and memory management. Windows 2.0 allowed application windows to overlap each other and also introduced more sophisticated keyboard-shortcuts. It could also make use of expanded memory.
Windows 2.1 was released in two different flavors: Windows/386 employed the 386 virtual 8086 mode to multitask several DOS programs, and the paged memory model to emulate expanded memory using available extended memory. Windows/286 (which, despite its name, would run on the 8086) still ran in real mode, but could make use of the high memory area.

The early versions of Windows were often thought of as simply graphical user interfaces, mostly because they ran on top of MS-DOS and used it for file system services. However, even the earliest 16-bit Windows versions already assumed many typical operating system functions; notably, having their own executable file format and providing their own device drivers (timer, graphics, printer, mouse, keyboard and sound) for applications. Unlike MS-DOS, Windows allowed users to execute multiple graphical applications at the same time, through cooperative multitasking. Windows implemented an elaborate, segment-based, software virtual memory scheme, which allowed it to run applications larger than available memory: code segments and resources were swapped in and thrown away when memory became scarce, and data segments moved in memory when a given application had relinquished processor control, typically waiting for user input.

Windows 3.0 and 3.1
Windows 3.0 (1990) and Windows 3.1 (1992) improved the design, mostly because of virtual memory and loadable virtual device drivers (VxDs) which allowed them to share arbitrary devices between multitasked DOS windows. Also, Windows applications could now run in protected mode (when Windows was running in Standard or 386 Enhanced Mode), which gave them access to several megabytes of memory and removed the obligation to participate in the software virtual memory scheme. They still ran inside the same address space, where the segmented memory provided a degree of protection, and multi-tasked cooperatively. For Windows 3.0, Microsoft also rewrote critical operations from C into assembly, making this release faster and less memory-hungry than its predecessors.With the introduction of Windows for Workgroups 3.11, Windows was able to bypass DOS for file management operations using 32-bit file access.

Windows 95, 98, and Me
Windows 95 was released in 1995, featuring a new user interface, support for long file names of up to 255 characters, and the ability to automatically detect and configure installed hardware (plug and play). It could natively run 32-bit applications, and featured several technological improvements that increased its stability over Windows 3.1. There were several OEM Service Releases (OSR) of Windows 95, each of which was roughly equivalent to a service pack.

Microsoft's next release was Windows 98 in 1998. Microsoft released a second version of Windows 98 in 1999, named Windows 98 Second Edition (often shortened to Windows 98 SE).
In 2000, Microsoft released Windows Me (Me standing for Millennium Edition), which updated the core from Windows 98 but adopted some aspects of Windows 2000 and removed the "boot in DOS mode" option. It also added a new feature called System Restore, allowing the user to set the computer's settings back to an earlier date.

Windows NT family
The NT family of Windows systems was fashioned and marketed for higher reliability business use. The first release was MS Windows NT 3.1 (1993), numbered "3.1" to match the consumer Windows version, which was followed by NT 3.5 (1994), NT 3.51 (1995), NT 4.0 (1996), and Windows 2000 (2000). 2000 is the last NT-based Windows release which does not include Microsoft Product Activation. NT 4.0 was the first in this line to implement the "Windows 95" user interface (and the first to include Windows 95’s built-in 32-bit runtimes). Microsoft then moved to combine their consumer and business operating systems with Windows XP, coming in both home and professional versions (and later niche market versions for tablet PCs and media centers); they also diverged release schedules for server operating systems. Windows Server 2003, released a year and a half after Windows XP, brought Windows Server up to date with MS Windows XP. After a lengthy development process, Windows Vista was released toward the end of 2006, and its server counterpart, Windows Server 2008 was released in early 2008. On July 22, 2009, Windows 7 and Windows Server 2008 R2 were released as RTM (release to manufacturing). Windows 7 was released on October 22, 2009.

64-bit operating systems
Windows NT included support for several different platforms before the x86-based personal computer became dominant in the professional world. Versions of NT from 3.1 to 4.0 variously supported PowerPC, DEC Alpha and MIPS R4000, some of which were 64-bit processors, although the operating system treated them as 32-bit processors.
With the introduction of the Intel Itanium architecture (also known as IA-64), Microsoft released new versions of Windows to support it. Itanium versions of Windows XP and Windows Server 2003 were released at the same time as their mainstream x86 (32-bit) counterparts. On April 25, 2005, Microsoft released Windows XP Professional x64 Edition and Windows Server 2003 x64 Editions to support the x86-64 (or x64 in Microsoft terminology) architecture. Microsoft dropped support for the Itanium version of Windows XP in 2005. Windows Vista is the first end-user version of Windows that Microsoft has released simultaneously in x86 and x64 editions. Windows Vista does not support the Itanium architecture. The modern 64-bit Windows family comprises AMD64/Intel64 versions of Windows Vista, and Windows Server 2008, in both Itanium and x64 editions. Windows Server 2008 R2 drops the 32-bit version, although Windows 7 does not.

Windows CE
Windows CE (officially known as Windows Embedded), is an edition of Windows that runs on minimalistic computers, like satellite navigation systems and, uncommonly, mobile phones. Windows Embedded runs as CE, rather than NT, which is why it should not be mistaken for Windows XP Embedded, which is NT. Windows CE was used in the Dreamcast along with Sega's own proprietary OS for the console. Windows CE is the core from which Windows Mobile is derived.

History
Microsoft has taken two parallel routes in its operating systems. One route has been for the home user and the other has been for the professional IT user. The dual routes have generally led to home versions having greater multimedia support and less functionality in networking and security, and professional versions having inferior multimedia support and better networking and security.
The first version of Microsoft Windows, version 1.0, released in November 1985, lacked a degree of functionality and achieved little popularity, and was to compete with Apple’s own operating system.Windows 1.0 is not a complete operating system; rather, it extends MS-DOS. Microsoft Windows version 2.0 was released in November, 1987 and was slightly more popular than its predecessor. Windows 2.03 (release date January 1988) had changed the OS from tiled windows to overlapping windows. The result of this change led to Apple Computer filing a suit against Microsoft alleging infringement on Apple's copyrights.


Microsoft Windows version 3.0, released in 1990, was the first Microsoft Windows version to achieve broad commercial success, selling 2 million copies in the first six months. It featured improvements to the user interface and to multitasking capabilities. It received a facelift in Windows 3.1, made generally available on March 1, 1992. Windows 3.1 support ended on December 31, 2001.

In July 1993, Microsoft released Windows NT based on a new kernel. NT was considered to be the professional OS and was the first Windows version to utilize preemptive multitasking.Windows NT would later be retooled to also function as a home operating system, with Windows XP.

On August 24, 1995, Microsoft released Windows 95, a new, and major, consumer version that made further changes to the user interface, and also used preemptive multitasking. Windows 95 was designed to replace not only Windows 3.1, but also Windows for Workgroups, and MS-DOS. It was also the first Windows operating system to use Plug and Play capabilities. The changes Windows 95 brought to the desktop were revolutionary, as opposed to evolutionary, such as those in Windows 98 and Windows Me. Mainstream support for Windows 95 ended on December 31, 2000 and extended support for Windows 95 ended on December 31, 2001.

The next in the consumer line was Microsoft Windows 98 released on June 25, 1998. It was substantially criticized for its slowness and for its unreliability compared with Windows 95, but many of its basic problems were later rectified with the release of Windows 98 Second Edition in 1999. Mainstream support for Windows 98 ended on June 30, 2002 and extended support for Windows 98 ended on July 11, 2006.
As part of its "professional" line, Microsoft released Windows 2000 in February 2000. The consumer version following Windows 98 was Windows Me (Windows Millennium Edition). Released in September 2000, Windows Me implemented a number of new technologies for Microsoft: most notably publicized was "Universal Plug and Play". During 2004 part of the Source Code for Windows 2000 was leaked onto the internet. This was bad for Microsoft as the same kernel used in Windows 2000 was used in Windows XP.

In October 2001, Microsoft released Windows XP, a version built on the Windows NT kernel that also retained the consumer-oriented usability of Windows 95 and its successors. This new version was widely praised in computer magazines. It shipped in two distinct editions, "Home" and "Professional", the former lacking many of the superior security and networking features of the Professional edition. Additionally, the first "Media Center" edition was released in 2002, with an emphasis on support for DVD and TV functionality including program recording and a remote control. Mainstream support for Windows XP ended on April 14, 2009. Extended support will continue until April 8, 2014.
In April 2003, Windows Server 2003 was introduced, replacing the Windows 2000 line of server products with a number of new features and a strong focus on security; this was followed in December 2005 by Windows Server 2003 R2.

On January 30, 2007 Microsoft released Windows Vista. It contains a number of new features, from a redesigned shell and user interface to significant technical changes, with a particular focus on security features. It is available in a number of different editions, and has been subject to some criticism.
Security
  
Consumer versions of Windows were originally designed for ease-of-use on a single-user PC without a network connection, and did not have security features built in from the outset. However, Windows NT and its successors are designed for security (including on a network) and multi-user PCs, but were not initially designed with Internet security in mind as much, since, when it was first developed in the early 1990s, Internet use was less prevalent.
These design issues combined with programming errors (e.g. buffer overflows) and the popularity of Windows means that it is a frequent target of computer worm and virus writers. In June 2005, Bruce Schneier’s Counterpane Internet Security reported that it had seen over 1,000 new viruses and worms in the previous six months.
Microsoft releases security patches through its Windows Update service approximately once a month (usually the second Tuesday of the month), although critical updates are made available at shorter intervals when necessary. In versions of Windows after and including Windows 2000 SP3 and Windows XP, updates can be automatically downloaded and installed if the user selects to do so. As a result, Service Pack 2 for Windows XP, as well as Service Pack 1 for Windows Server 2003, were installed by users more quickly than it otherwise might have been.

While the Windows 9x series offered the option of having profiles for multiple users, they had no concept of access privileges, and did not allow concurrent access; and so were not true multi-user operating systems. In addition, they implemented only partial memory protection. They were accordingly widely criticised for lack of security.

The Windows NT series of operating systems, by contrast, are true multi-user, and implement absolute memory protection. However, a lot of the advantages of being a true multi-user operating system were nullified by the fact that, prior to Windows Vista, the first user account created during the setup process was an administrator account, which was also the default for new accounts. Though Windows XP did have limited accounts, the majority of home users did not change to an account type with fewer rights – partially due to the number of programs which unnecessarily required administrator rights – and so most home users ran as administrator all the time.

Windows Vista changes this by introducing a privilege elevation system called User Account Control. When logging in as a standard user, a logon session is created and a token containing only the most basic privileges is assigned. In this way, the new logon session is incapable of making changes that would affect the entire system. When logging in as a user in the Administrators group, two separate tokens are assigned. The first token contains all privileges typically awarded to an administrator, and the second is a restricted token similar to what a standard user would receive. User applications, including the Windows Shell, are then started with the restricted token, resulting in a reduced privilege environment even under an Administrator account. When an application requests higher privileges or "Run as administrator" is clicked, UAC will prompt for confirmation and, if consent is given (including administrator credentials if the account requesting the elevation is not a member of the administrators group), start the process using the unrestricted token.

File permissions
All Windows versions from Windows NT 3 have been based on a file system permission system referred to as AGLP (Accounts, Global, Local, Permissions) AGDLP which in essence where file permissions are applied to the file/folder in the form of a 'local group' which then has other 'global groups' as members. These global groups then hold other groups or users depending on different Windows versions used. This system varies from other vendor products such as Linux and NetWare due to the 'static' allocation of permission being applied directory to the file or folder. However using this process of AGLP/AGDLP/AGUDLP allows a small number of static permissions to be applied and allows for easy changes to the account groups without reapplying the file permissions on the files and folders.

Windows Defender
On January 6, 2005, Microsoft released a Beta version of Microsoft AntiSpyware, based upon the previously released Giant AntiSpyware. On February 14, 2006, Microsoft AntiSpyware became Windows Defender with the release of Beta 2. Windows Defender is a freeware program designed to protect against spyware and other unwanted software. Windows XP and Windows Server 2003 users who have genuine copies of Microsoft Windows can freely download the program from Microsoft's web site, and Windows Defender ships as part of Windows Vista and 7.

Third-party analysis
In an article based on a report by Symantec, internetnews.com has described Microsoft Windows as having the "fewest number of patches and the shortest average patch development time of the five operating systems it monitored in the last six months of 2006."
A study conducted by Kevin Mitnick and marketing communications firm Avantgarde in 2004 found that an unprotected and unpatched Windows XP system with Service Pack 1 lasted only 4 minutes on the Internet before it was compromised, and an unprotected and also unpatched Windows Server 2003 system was compromised after being connected to the internet for 8 hours. However, it is important to note that this study does not apply to Windows XP systems running the Service Pack 2 update (released in late 2004), which vastly improved the security of Windows XP. The computer that was running Windows XP Service Pack 2 was not compromised. The AOL National Cyber Security Alliance Online Safety Study of October 2004 determined that 80% of Windows users were infected by at least one spyware/adware product. Much documentation is available describing how to increase the security of Microsoft Windows products. Typical suggestions include deploying Microsoft Windows behind a hardware or software firewall, running anti-virus and anti-spyware software, and installing patches as they become available through Windows Update.

source : en.wikipedia.org