I just cleaned up a computer for a friend, their kids had completely trashed it with spyware and viruses. She had a lot of her work data on there (not backed up, of course) so she was pretty frantic about recovering it. Dell tech support said the only fix would be the restore CD's (there's a known fix now, the main problem was "parser message value creation failed at line 422").
Anyways, here's the story:
She had McAfee installed, it detected nothing.
I pulled the drive and scanned it with Norton 2004 (it wasn't installed as a boot drive), NAV found and cleaned 6 different viruses.
I reinstalled the drive in her computer, then did an online scan at the Panda website, this found 8 more which I had to clean up manually.
Finally, I uninstalled the McAfee, put on my copy of AVP 4.5 Pro, and it cleaned out 11 more.
This isn't a completely valid comparison, since the programs were being used under a variety of conditions, but I'm VERY happy with AVP, I've had it catch viruses that Norton never seems to catch, so I've put AVP on all 3 computers now and haven't had any trouble at all.