The Microsoft Security Essentials protects from viruses and malware and has been rated pretty good by the PC industry.
And it's free, as long as you have a valid XP/Vista/Win7 license.
http://www.microsoft.com/Security_Essentials/
Review:
Microsoft Security Essentials 1.0 - Full Review - Reviews by PC Magazine
I've been using it and installing it on friends and family's computers as well...