I think Avast is the best free virus scanner but I recently replaced it and ZoneAlarm with the NOD32 Smart Security suite - virus scanner, firewall, spyware blocker, spam blocker. So far it has been excellent and is very easy on the resources.
Another good free one is ThreatFire. It scans based on behaviour instead of a signature file so it can catch 0-day viruses. I run it along side NOD32 without problem.
Check this site for AV ratings:
http://www.av-comparatives.org/