Many will say AVG, but through the questions I have answered here and on my blog I do not believe that V8 is as good as its predecessors.
Personally, I use and recommend Avast anti-virus better definitions, anti-spyware and anti-rootkit built in, faster and more reliable:
http://www.download.com/Avast-Home-Edition/3000-2239_4-10019223.html?part=dl-AvastHome&subj=dl&tag=button&cdlPid=10852750
Setup Guide:
http://cotojo.wordpress.com/2008/07/15
Features include Anti-spyware, Anti-rootkit, Antivirus kernel, Automatic updates, Integrated Virus Cleaner, International Languages, Network Shield, P2P and IM Shields, Resident protection, Simple User Interface, Strong self-protection, Support for 64-bit Windows Systems, System integration.
Low resources and regular updates of program and definition files.