Nortons is the only antivirus that has run perfectly for me (well, 2002 anyway, 2004 was a huge mess), but that's not free and I'm stingy. =]
Heh. Out of all anti-virus programs, the one I have the worst experiences with is Norton antivirus (or at least, the one bundled with 'Norton Internet Security'). Easily the most craptacular antivirus program to ever roam the planet. continuously chokes on large email messages, gobbles up oodles or resources, and slows down your system to a crawl.
Running either Norton or no virus scanner at all is a tough choice. ;-)