The Register reminds us that Microsoft's support for Windows XP will cease to exist two years from now, on April 8th, 2014:
XP was shipped to OEMs on August 24th, 2001 and reached average punters on October 25th.
Plenty bought it and plenty still run it: Gartner's July 2011 assessment of the global OS population suggested "Windows XP Home and Follow-Ons" had 68 million users, while XP Professional ran on 144 million machines.
A more recent Gartner study, the March 2012 Client OS and Office Survey reported 79% of business desktops and 45% of notebooks ran XP, based on responses from a 147-strong, self-selecting, group at its October 2011 US Symposium event. While the analyst firm notes that's not the most scientific of samples, the respondents represented organisations with a combined three million PCs in service.