It would be best to stay well clear of Norton and McAfee Antivirus. They cause problems all the time, interfering in with startup, windows update, sidebar, games and they use more resourceses than alot of other antivirus programs available.
Zonealarm has been rated the best on many sites. Although it is slower then the rest, it is the most through. It catches pretty much any virus and stops it in its tracks. Go here if you want to see for yourself.