First comment I have on this is: THIS SUCKS!
Why can't we not have Halo 2 on WinXP? Yes, I know it should use DX10. This is just a Microsoft marketing scheme to get people to splash out and buy Vista! The biggest problem is hardware. In order to play/use Vista properly you will need an up to date machine which probably mean a 50GHz processor machine with 2 TB memory! Ok, that was just a joke but you get the message.
I've been waiting to play this game for a long time and now it's even going to be longer and harder to get. Anyway here is the *cough* slashdot article on it.
http://games.slashdot.org/games/06/02/09/2010238.shtml