AVG used to be the only free solution. It works, but has its reliability issues.
A free professional solution - Avast 32 - has been free for over a year now for home users and has obtained 100% detection rates in viruses in the wild. It is currently the best free AV solution for home users and I hope it stays that way. Their logic is, if you use it at home, you will use it at work too.
It is also updated daily.
-S
http://www.avast.com/