I (being the security-nut-insane-geek-nerd) have tried and used almost all the antvirus programs out there, so here I list the popular ones off of the top of my head (with ratings, of course) - I may make a complete list later.
Norton Systemworks 2001: 2/5
Norton Systemworks 2003: 2.5/5
Avast! Home Edition: 4/5
AVG Free Edition: 4.5/5
BitDefender 8 Free Edition: 4/5
AntiVir PersonalEdition Classic: 4/5
McAffee Internet Security Suite: 4/5
Windows Live OneCare: I am using this now and it is not the best (needs more options, but is still in heavy development) but still far from the worst (sure beats Avast! and BitDefender off the list - Norton and McAffee are always off the list anyways), but I love the fact that I can blame Microsoft for a problem
...well not really. But here is the run down on it and why I like it:
It scans for Viruses AND spyware both on-demand and silently (if you choose to install Windows Defender - previously Microsoft Antispyware).
It defragments and cleanes your hard drives both on-demand and silently.
It Backs-Up and restores your files both on-demand and silently.
It can be installed on 3 PC's for 1 year for only $50.
And if that isn't enough, they even offer an online FREE version, a 90 day free trial period (WITH Tech support via email) and if you purchase it and need help you can chat online, email them or talk over the phone!
Anyways, enough about Windows stuff.
Basically if you need high-end security for free, go to AntiVir PersonalEdition Classic or AVG Free. If you need more, try WLOC or ZoneLabs solutions.