This is an obvious move by Microsoft to get people to buy Windows Vista.  Halo 2 could have been very easily been made for Windows XP but no we will instead make you buy a new OS to be able to play a 3 year old game.  Besides getting a Xbox and Halo 2 would be half the price.  Its just outrageous and what is even more so is that many will decide to get Vista just because of this.