AVG Free Edition:
Are u looking for a very good anti virus software for gratis? Then u need AVG free edition. I've been using it since version 7.0 and now with version 8.0 it provides effective protection against virus, malwares, spywares, trojan horses. It has an active anti-virus, anti-spyware, E-mail scanner, Update manager. Just install it and forget everything else. It also doesn't slow the system down, so try it yourself.
For an antivirus software, avg is fantastic. But why let a virus access your pc at all? That's the job of a firewall and i assure that i ain't talking about the windows one. There're a lot of free good firewalls, but i'm speaking about comodo. It's nice and compact, checks the system at initial installation, and prevents any application that tries to run at the background. Ever since i installed avg and comodo together, there've not been any virus related problems at all. So give it a shot, u might even like it.