Actually, Windows XP is the best selling version of Windows ever (maybe any OS). From Wikipedia: According to web analytics data generated by W3Schools, as of

Actually, Windows XP is the best selling version of Windows ever (maybe any OS). From Wikipedia: According to web analytics data generated by W3Schools, as of June 2011, Windows XP is the most widely used operating system for accessing the Internet in the world with a 39.7% market share, having peaked at 76.1% in January 2007.