http://www.modacity.net/forums/announcement.php?f=27

Quote Originally Posted by me
That said, any threads made after the date of this announcement (Dec. 15, 2007 at 4:30am Eastern Time) asking if the game will run on their computer will be given an "Unconstructive Post" infraction (2 points, 21 days to decay) and the thread will be locked and/or deleted. Also, anyone asking for help with running the game on Windows XP via WoWloader or any kind of Windows Vista environment emulation patch will also result in the same treatment.
It's been proven time and time again that Halo 2 will not work on XP for a long time. While we welcome people attempting to hack the game so it will run on XP with no problem, the current patches that exist can only barely get singleplayer to run on XP, and this is why we do not support this. If you want to play Halo 2 for Windows Vista, then simply buy Windows Vista and stop complaining.