I've been running Kaspersky 2009 Internet Security without any problems on Windows 7 and have it on Vista too. It seems to do well in reviews and shopping around I found I could buy it for 17GBP which is about half the price of the average game (Like Left4Dead which is awesome!) but the BEST bit is that that was for a 3 PC licence.
What I personally like about Kaspersky is that it updates it's definitions every 2 hours, but actually rating antivirus programs is a very difficult thing. If they do their job, the best you can hope for is that you don't ever notice them and nothing ever happens..... it's hard to say what's the best at making nothing happen.
As mentioned by Nighthawk, most of them offer trials, go and have a play and see which one you like the most. Just don't download any trials from dodgy sites like
www.hackintomypcandwreckit.com (I just made that up.)