for the people who can't uninstall halo2 under winXP, this problems got me too. Fortunately, I have my system running dual boot. WinXP and Vista. First time, I want to try it in WinXP. been success in installing the game thru loader, but it won't play. Everytime I choose the campaign, it crashes with a warning dialog. and when i want to uninstall it from add/remove programs, it just splash and then nothing happens. I managed to work it out:
1. delete the folder where halo2 was installed
2. delete the folder Halo2 under my documents\my games or search your hard drive for halo2 folder, and then manually deleted it
3. open regedit (start-->run-->regedit-->enter), and hit F3 to search halo2 in the entire registry. if you found it, delete it, but be careful as some registry search will gives you something like: xxxxxhalOxxxx. do not delete this key!
4. to delete the entry in the add/remove programs, I use RegCleaner (it's a freeware. google it), and navigate the uninstall menu, mark the check box under halo2, and click delete.
5. restart your PC
hopes it helps
Now, I play it under vista (where it shuld be played)