I recommend you try Unthreat, it's a free antivirus and it's easy to use. It updates it's virus DB regularly and doesn't slow down my computer while running. I haven't had a virus since I downloaded it so it gets the job done.
I'm surprised no one has mentioned Microsoft Security Essentials. I know, I know, it's Microsoft blablablah and all but honestly, *knock wood* my system has been clean since I've been using it a year and a half ago. And yes, I was using Avast and Avira prior to my SE discovery.