We use the corporate version of AVG. For home use there's a free version at: http://free.grisoft.com
Also heard good reports of NOD32 and Trend Micro.
Avoid like the Plague of the Ori: Norton.
Also note that few AV products detect spyware, which is a bigger problem
than viruses these days. Thus you need a spyware-scanner too, such as
Ad-Aware: http://lavasoftusa.com