Microsoft Security Essentials - http://windows.microsoft.com/en-us/windows/security-essentials-download - is free, quite light and actually good enough.
Howver, the story doesn't end there (as most
/. readers already probably know.... Inman put up a fundraising website to raise $20 000 to donate to cancer research (and bear love) - http://www.indiegogo.com/bearlovegood - where he has up til now raised - brace yourself - more than $ 100 000 !!!