+1 for Avira. Don't know why people are still using AVG(Free, which is based off of the same engine as the paid version), it isn't that great any more. I switched to Aviria after it found a few things AVG missed.
AVG actually tried to tell me NetBeans and .NET updates (Java IDE, downloaded from www.netbeans.org and .NET updates via Windows Update) was a virus! lol. It also told me that my host file (Located in: C:\WINDOWS\system32\drivers\etc) was a virus, which wasn't.
Best Free Anti-Virus software: There was no contest with the freebie –Antivir was above and beyond its competition. Enough said.