The best anti-virus solution is a smart user. Don't use Internet Explorer, use Firefox, Opera, or Chrome. Firefox has an amazing array of addons (Ad block+, No Script, etc). Make sure you know what you're downloading and running.
As far as software goes, AVG is decent. Both Norton and McAfee are crap, unless you get the corporate versions but even those aren't that great. NOD32 and BitDefender are both the best, in my opinion. Kaspersky is a resource hog and I didn't care for it. NOD32 and BD are both lightweight and offer excellent protection.
If you don't want to spend money, get AVG. Personally, I don't run virus software since I just watch what I do and only I use my PC.
Edit: There's also AntiVir Personal, I think it's free, and it's quite good.