Windows XP, the successor to Windows 2000 and Windows ME, was the first consumer-oriented operating system produced by Microsoft to be built on the Windows NT kernel. Windows XP was released worldwide for retail sale on October 25, 2001, and over 400 million copies were in use in January 2006. It was succeeded by Windows Vista in January 2007. Direct OEM and retail sales of Windows XP ceased on June 30, 2008. Microsoft continued to sell Windows XP through their System Builders (smaller OEMs who sell assembled computers) program until January 31, 2009. On April 10, 2012, Microsoft reaffirmed that extended support for Windows XP and Office 2003 would end on April 8, 2014 and suggested that administrators begin preparing to migrate to a newer OS.
The NT-based versions of Windows, which are programmed in C, C++, and assembly, are known for their improved stability and efficiency over the 9x versions of Microsoft Windows. Windows XP presented a significantly redesigned graphical user interface, a change Microsoft promoted as more user-friendly than previous versions of Windows. In an attempt to further ameliorate the "DLL hell" that plagued the past versions of Windows, improved side-by-side assembly technology in Windows XP allows side-by-side installation, registration and servicing of multiple versions of globally shared software components in full isolation. It is also the first version of Windows to use product activation to combat illegal copying.
During Windows XP's development, the project was codenamed "Whistler", after Whistler, British Columbia, as many Microsoft employees skiied at the Whistler-Blackcomb ski resort.
According to web analytics data generated by Net Applications, Windows XP was the most widely used operating system until August 2012, when Windows 7 overtook it. As of August 2013, Windows XP market share is at 33.66%, having decreased almost every month since at least November 2007, the first month for which statistics are publicly available from Net Applications.