It seems that it is possible to run HALO 2 (PC) on Windows XP. The game is supposed to be exclusive to Windows Vista, however a gamer has made a rough and dirty mod to get the game running on Windows XP.

"Tired of waiting for Falling Leaf to produce drivers so that you can playShadowrun in XP? The wait is over, because Razor1911 already has the remedy! Yes, you read right. This game will also function in Windows XP".

Sources:
The Enquirer
halo.bungie.org

Download mod and instructions:
Download the mod