I don't know why but Avast seems to be the best for me these days. I have tried avg,nod 3 and others. Everyone who i recommend avast too thanks me for saving their computer. I have found on 3 computers avast the only anti virus to find things.
I switched from AVG to Avast about a year ago. Seems to work fine. Definitely less of a resource hog than AVG. Heck, it even found some nasties.
It's good. So is Avira (better detection rates). Nod32 is without a doubt the best, though. I am using it, used all the others, and it's in a separate league.
Avast is awesome. Low on resources and they have nice skins. I'm using it now. Used to have Avira installed until I got sick of the lame umbrella icon.
I've used most of them. The best free one at present seems to be antivir (fast, efficient, detection rate).