Black Ops 2 is by no means the first major PC game not to support XP. Sleeping Dogs, which was released last month, is a recent example of another game to spurn the OS. Most high-profile games, however, do still support Windows XP, mainly because a significant number of gamers still use it. Steam's August Hardware and Software Survey reveals 13% of its audience ran their games on Windows XP, although that's compared to the 71% who used Windows 7.
The Black Ops 2 news coincides with Windows 7 narrowly overtaking XP as the most used desktop OS, according to NetMarketShare (not forgetting, of course, the launch of Windows 8 next month). XP is just coming up to its 11th birthday, but the overall picture suggests the venerable OS may be on its last legs, at least when it comes to gaming.
Microsoft Windows 8
Microsoft Windows XP