AVG Free 8.5 is the best free anti-virus you can get. Avast used to be good, but over time AVG simply grew on me. Besides, Avast used to take a long time to scan every mp3/avi file, depending on the file size. Eg. a 7mb mp3 would take about 30 seconds to scan and similarly a 700mb avi would take about 2 minutes. This maybe because i was using a pentium 4 at that time, but even then, when i switched to avg, the problem ceased to exist. All other services are a limited free trial period offer
AFAIK.
AVG>Avast. If you can find any other free ones, let me know. I want to test some free ones too.