Windows XP is a version of the Microsoft Windows operating system for personal computers. The letters "XP" stand for "eXPerience". Microsoft released Windows XP on October 25, 2001. Windows XP replaced Windows 2000 and Windows ME, which helped complete the unification of the NT and 9x branches of Windows. It was replaced by Windows Vista. Windows XP was the second most used computer operating system in the world as late as April 2012. Following the release of Windows XP, many computer manufacturers including (but not limited to) Dell, Hewlett Packard, Acer, IBM, Compaq, and Toshiba rebadged their computers (mostly laptops) to Windows XP from Windows 2000. You can tell a laptop from that time was initially sold with Windows 2000 if the Windows (WIN) key on the keyboard has the previous Windows logo.