Windows XP is a line of operating systems produced by Microsoft for use on personal computers, including home and business desktops, notebook computers, and media centers. The name "XP" is short for "experience". Windows XP is the successor to both Windows 2000 Professional and Windows Me, and is the first consumer-oriented operating system produced by Microsoft to be built on the Windows NT kernel and architecture. Windows XP was first released on October 25, 2001, and over 400 million copies were in use in January 2006, according to an estimate in that month by an IDC analyst. It was succeeded by Windows Vista, which was released to volume license customers on November 8, 2006, and worldwide to the general public on January 30, 2007. Direct OEM and retail sales of Windows XP ceased on June 30, 2008. Microsoft continued to sell XP through their System Builder Channel (smaller OEMs who sell assembled computers) until January 31, 2009. XP may continue to be available as these sources run through their inventory or by purchasing Windows Vista Ultimate or Business and then downgrading to Windows XP.
The most common editions of the operating system are Windows XP Home Edition, which is targeted at home users, and Windows XP Professional, which offers additional features such as support for Windows Server domains and two physical processors, and is targeted at power users, business and enterprise clients. Windows XP Media Center Edition has additional multimedia features enhancing the ability to record and watch TV shows, view DVD movies, and listen to music. Windows XP Tablet PC Edition is designed to run stylus applications built using the Tablet PC platform. Windows XP was eventually released for two additional architectures, Windows XP 64-bit Edition for IA-64 (Itanium) processors and Windows XP Professional x64 Edition for x86-64. There is also Windows XP Embedded, a component version of the Windows XP Professional, and editions for specific markets such as Windows XP Starter Edition. By mid 2009, a manufacturer revealed the first Windows XP powered cellular telephone.
The NT-based versions of Windows are known for their improved stability and efficiency over the 9x versions of Microsoft Windows. Windows XP presents a significantly redesigned graphical 
user interface, a change Microsoft promoted as more user-friendly than previous versions of Windows. A new software management facility called Side-by-Side Assembly was introduced to ameliorate the "DLL hell" that plagues 9x versions of Windows. It is also the first version of Windows to use product activation to combat illegal copying, a restriction that did not sit well with some users and privacy advocates. Windows XP has also been criticized by some users for security vulnerabilities, tight integration of applications such as Internet Explorer 6 and Windows Media Player, and for aspects of its default user interface. Later versions with Service Pack 2, Service Pack 3, and Internet Explorer 8 addressed some of these concerns.
During development, the project was codenamed "Whistler", after Whistler, British Columbia, as many Microsoft employees skied at the Whistler-Blackcomb ski resort.
As of the end of June 2009, Windows XP is the most widely used operating system in the world with a 66.9% market share, having peaked at 76.1% in January 2007. According to a Net Applications report, the Windows XP market share peaked as high as 85.3% in December 2006.
To try to curb piracy based on leaked or generated volume license keys, Microsoft introduced Windows Genuine Advantage (WGA). WGA comprises two parts, a verification tool which must be used to get certain downloads from Microsoft and a user notification system. WGA for Windows was followed by verification systems for Internet Explorer 7, Windows Media Player 11, Windows Defender, Microsoft Office 2007 and certain updates. Recently Microsoft removed the WGA verification from the installer for Internet Explorer 7 saying that the purpose of the change was to make IE7 available to all Windows users.
If the license key is judged not genuine, it displays a nag screen at regular intervals asking the user to buy a license from Microsoft. In addition, the user's access to Microsoft Update is restricted to critical security updates, and as such, new versions of enhancements and other Microsoft products will no longer be able to be downloaded or installed. As of August 26, 2008, Microsoft has released a new WGA activation program that displays a plain black desktop background for computers failing validation. The background can be changed, but reverts after 1 hour.
Common criticisms of WGA have included its description as a "Critical Security Update", causing Automatic Updates to download it without user intervention on default settings, its behavior compared to spyware of "phoning home" to Microsoft every time the computer is connected to the Internet, the failure to inform end users what exactly WGA would do once installed (rectified by a 2006 update), the failure to provide a proper uninstallation method during beta testing (users were given manual removal instructions that did not work with the final build), and its sensitivity to hardware changes which cause repeated need for reactivation in the hands of some developers. Also if the user has no connection to the internet or a phone, it will be difficult to activate it normally.
Strictly speaking, neither the download nor the install of the Notifications is mandatory; the user
can change their Automatic Update settings to allow them to choose what updates may be downloaded for installation. If the update is already downloaded, the user can choose not to accept the supplemental EULA provided for the Notifications. In both cases, the user can also request that the update not be presented again. Newer Critical Security Updates may still be installed with the update hidden. However this setting will only have effect on the existing version of Notifications, so it can appear again as a new version. As of 2006, Microsoft was involved in a class action lawsuit brought forth in California, on grounds that it violated the spyware laws in the state with its Windows Genuine Advantage Notifications program.
Original Equipment Manufacturer (OEM) licenses are preinstalled on, and sold with, pre-assembled computers from system manufacturers. There are two types of OEM product types — those used for "Direct OEMs" (major name brands that buy through a direct contract with Microsoft and produce and brand their own media from a Microsoft "Gold Master Copy" by using an Authorized Microsoft Duplication Partner), and those used for "System Builders" (local computer shops that buy generic, unbranded kits through Authorized Microsoft Distributors). Direct OEM product keys will often not activate with System Builder installation media because Direct OEMs are now required by Microsoft to pre-activate their copies in the factory using their own internal mechanism before delivery to the customer. It is recommended that System Builders also pre-activate their systems before delivery, but this is not mandatory.
OEM installations can be customized using the Microsoft OEM Preinstallation Kit with branding, logos, additional applications, optional services, alternate applications for certain Windows components, Internet Explorer links, and various other customizations. All OEM customers must include support and contact information for the initial installation of Windows because it is the responsibility for the OEM to support the Windows installation, and is not provided by Microsoft to the end-user. Direct OEMs must create their own media, but have the option of creating their own custom recovery solution, which may or may not be similar to a generic installation. OEMs may provide a recovery partition on the hard drive as the custom recovery solution rather than providing disc-based media with the computer.
Some end-users have found this to be a troublesome option, because in the event of an out-of-warranty hard drive failure, they may not have access to reinstall Windows on a new hard drive.
System Builders are not allowed the option to create a custom recovery CD/DVD media. The only deliverable media available for a System Builder to give to the end-user is the unbranded OEM System Builder hologram media kit. Because of this, when end-users reformat their hard drives and re-install from the installation media, they lose all the custom branding and support information that the System Builder would have included.
As a supplemental recovery method to a CD/DVD-based installation, a System Builder may employ a fully customized recovery solution on the hard drive. Whether utilizing a recovery partition or not, a System Builder must still include the original generic OEM System Builder hologram CD/DVD media kit. OEM licenses are not transferable from one computer to another. Every computer sold/resold with an OEM license must include all of the original installation media or recovery solution, documentation, Certificate of Authenticity, and product key sticker with the sale. Microsoft requires that all OEM system manufacturers include as part of the configuration the Windows Out-of-Box Experience (OOBE), which is the initial setup wizard encountered the first time Windows boots-up. It is also required that Value-Added Resellers (VAR's), retailers, and general resellers not tamper with the OEM's customized OOBE mechanism unless under permission by the OEM, and it is a recommended configuration for systems that are privately resold so that a customer will have a like-new computer experience upon first boot-up.
OEM licenses are to be installed by professional system manufacturers only. Under Microsoft's OEM License Agreement, they are not to be sold to end-users under any circumstance, and are to be preinstalled on a computer using the OEM Preinstallation Kit (OPK) before shipment to 
the customer, and must include at the very least the manufacturer's support contact information. They are, therefore, designed for installation only on a single computer and are not transferable, even if the original computer is no longer in use. This is not usually an issue for users who purchase new computer systems, because most pre-assembled systems ship with a preinstalled operating system. There are few circumstances where Microsoft will allow the transfer of an OEM license from one non-functioning system to another, but the OEM System Builder License Agreement (SBLA), as well as the OEM End User License Agreement (EULA) do not contain any allowance for this, so it is entirely up to Microsoft's discretion, depending on the situation.
user interface, a change Microsoft promoted as more user-friendly than previous versions of Windows. A new software management facility called Side-by-Side Assembly was introduced to ameliorate the "DLL hell" that plagues 9x versions of Windows. It is also the first version of Windows to use product activation to combat illegal copying, a restriction that did not sit well with some users and privacy advocates. Windows XP has also been criticized by some users for security vulnerabilities, tight integration of applications such as Internet Explorer 6 and Windows Media Player, and for aspects of its default user interface. Later versions with Service Pack 2, Service Pack 3, and Internet Explorer 8 addressed some of these concerns.
