VoodooExtreme reports about Halo 2 problems in Windows Vista with the UAC feature:
During recent testing of Halo 2, a game developed by Microsoft and built exclusively for Vista, several issues were found. Vista's User Account Control (UAC), a security feature intended to make the OS safe, has become the number one most hated new feature. As the UAC tries to verify every program, in some cases numerous times, with an endless barrage of highly annoying "Do you want to install/run this program" windows. Virtually every power user and gamer deactivates the UAC within 5 minutes after installing Vista.. Sadly, it requires the UAC be enabled in order for Halo 2 to operate. Furthermore, the game will not run until one completes the registration for a Games for Windows - Live! account (more on this later). Simply bypassing the process causes the game to shut down.