According to system specifications that were released earlier today for Call of Duty: Black Ops 2, the upcoming shooter will not be supporting Windows XP. It's one of the first major titles released that will not support the operating system.
So if you're one of those 13% of gamers who got shellshocked by the disaster that was Windows Vista, and decided to stick with XP for the next decade, it's time to come out of your hiding place now. Or you could just not buy Black Ops 2. However, it's only a matter of time until XP support is dropped from all major games.
Black Ops 2 is not the first title to opt not to support the still-popular OS. Battlefield 3 and Sleeping Dogs both abandoned support for XP as well. Even Microsoft sometimes doesn't even support XP.