jcap and a few other standouts in the Halo PC and Halo 2 PC community have put their heads together and nailed down why Halo 2 for Windows Vista is a mess. I totally agree with these guys. Go read what they had to say: http://www.h2vista.net/forums/showthread.php?t=6854

Unfortunately, I can say that Shadowrun and now Gears of War (PC) are being handled the same way. Shadowrun, which is a mod of Halo 1 and was designed by Halo and Halo 2's lead designer, was a jewel of a game. It is my favorite first person shooter of all time. Unfortunately, the way Microsoft handled the game ruined the game for me and others. Halo 2 (PC) was the same way as you can read in the article above.

And guess what's happening now? Gears of War (PC) is in the same boat:

I having begun playing Gears of War (PC). It is riddled with problems just like Halo 2 (PC) and Shadowrun. I hope that the community will continue to express their disgust with the title support and Games for Windows - LIVE.