|Watch out for no-name brand free software as many are malware/spyware programs themselves.|
AVG is your best bet...
It's license is free for home users and the antivirus protection is as good as the commercial version, and when I was a partner in a computer store the bench techs found it to be better at virus protection than Norton.
The free version doesn't come with SPAM control, firewall, rootkit or spyware removers. But you can use other free software (Windows built in firewall, Ad-Aware, Malwarebytes Anti-Malware) to make up for that. And using safe browsing habits and common sense should help keep you clear of most threats.
Remember, there's no solution, commercial or free, that will 100% protect you from yourself.
Assume that I already did an Internet search.