I've heard Avast is very good and thus use it myself. Malwarebytes is a nice clean-up tool as well.
AvastMy best antivirus is intellectIts this thing that stops you from going to stupid places!
Microsoft Security Essentials.
PCWorld says Avira Anti Vir is the best free antivirusI use it in conjunction with Threatfire.Works like a charm.I've also heard that Malwarebytes is a good cleanup tool.
Spyware Search & Destroy