dwmapi.dll appears to be the file that specifies Halo2 as a vista only game. Seeing as you mentioned vista in your post, I'm assuming that you are in fact use that as your OS. Assuming that you haven't been trying to play on XP, you should probably just try re-installing the game.
If that doesn't work, I would try making the game XP compatible. I'm not sure if I am allowed to post how to do that on these forums, but I'm sure that you can find out how to do that by doing a quick google search ^^ Anyway, if you make it compatible with xp, you will be removing the file that is giving you a hard time which MAY fix your problem, gl ^^