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 25 October 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 8 November 2006, and worldwide to the general public on 30 January 2007. Direct OEM and retail sales of Windows XP ceased on 30 June 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.

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.