For those who don't know:
Well, I was at my friend's house and he told me that...my other friend...was on Halo 2 without having Vista and he said that if you pre-ordered Halo 2 Vista you get something to make it compatible for XP. (I don't know the exact words but that's what he said) I looked into it and here's the page I guess he found. Also, I heard that later on they are going to be releasing this for retail. I'm trying to find that now, if I do I'll edit this post. Just forget that for now.
P.S. I left out "my other friend" because if I mentioned his name it might start a flame war. Anyways, just ignore it.