I used http://www.tweakguides.com/Halo_1.html to help. Yes, that's for Halo 1 but many of the same issues and tweaks apply.
-> The most common issue I run into is a non "GAMING" machine. Most people don't have machines ready for games.
Halo 2 for Windows Vista has mouse control issues for some users. With VSYNC turned on for the video card, the mouse is laggy (doesn't respond for a second or more) and with VSYNC off the mouse is excruciatingly slow and stuttery. Microsoft support told me that more people need to complain about this issue before they will patch the game. I'm urging people who have the game and these issues to phone Microsoft (1-800-MICROSOFT) and complain asking for a resolution.