I use kaspersky Internet Security, which includes thier anti virus program, and it works great for me. Their anti-virus is supposed to be one of the best. This is a bit off topic, but i'm in love with kaspersky's Firewall that is included in the internet security suite. I must say, though, that if you are not technicaly savy, dont attempt to use it, because it is a bit complicated, unless you know what you are doing.
Also, I would like to add that Norton internet Security 08 is getting great reviews. They are progressively decreasing the processing power used by the program, without sacrificing any protection capability. Actually, i've also read that NIS 08 has improved protection on all fields. The only thing with NIS is that you would have to dish out about $70 yearly subscription, but it could be well worth it considering what's at stake. Kaspersky, at about $80, is a bit more expensive, but in my opinion, more effective. Norton is more user freindly.
In the end, it all depends on what you do and where your standards are. If you are not a hardcore networker/hacker/downloader, something like NIS would be good for you. If you are a hardcore networker/hacker/downloader and so on, you would need more advanced tools.
As far as free anti-virus programs go, in my opinion AVG comes first, then Avast. Those, by far, are the best two FREE tools out there right now.