From what I've heard, supposably the Security Accounts Control on Win7 is one of the best, if not best virus prevention system when its on maximum protection because it often freezes your computer's processes before anything can happen. Unfortunately its also annoying as hell. Imagine having to click a confirmation popup every 15 seconds for everyything you do. "Computer is trying to display start menu, Allow?", its terrible. But that super security means nothing if you keep mindlessly clicking "Allow" without reading what it is asking you.
It also slows down installing games. When I tried installing Ragnarok, UAC would scan the entire 1GB installer in the background, so if I double clicked the installer, I would wait like 4 minutes before the actual program window would show up.
But honestly I don't really know for sure if any of this is true. I know it was true for Windows Vista, but Windows 7 and many of its features are built/improved from Vista. I'm just assuming that maximum UAC on Win7 is the same as the UAC on Vista.
Anyways I don't use an antivirus, I'm quite confident in my ability of being my own virus protection. Besides the bug I got from torrenting 2 weeks ago, I haven't caught a virus since like 2003.