free stuff is the only way to go, IMO.
i personally, when i was running windows, preferred antivir. http://filehippo.com/download_antivir/
avg 8 is really good, but much more of a hog than 7.5.
avast 4.x is pretty good too. although, i think it has less bells and whistles than avg. but it is a better prog and less of a resource hog.
antivir is the least robust in terms of options. but less is more here. less false positives. better detection. the only thing that i have found that it is lacking is an email scanner. but i don't generally use outlook/thunderbird/eudora. so that's not an issue for me. could be for others.
i'm recommending antivir for a/v. combo of a-squared and spybot for anti-spy. comodo firewall for firewall. that should be a very nice suite of free progs that will keep you clean like a jimmy hat!