I'm behind an hardware firewall and my rule is "don't open it if you're not sure" so I feel quite safe, last time I got a "proper" virus was in 1997, more lately there were a couple of "trojans" that came in the mail and Avast home edition did a great job to keep them at bay, I run once in a while the Microsoft app for spyware and it always reports a clean system so Avast is sure doing a nice job.
I really liked Norton antivirus but it is a resource sucker and when the free subscription plan expired I simply uninstalled it.
Avast is more than enough, otherwise you could try Avira, everybody seem to say good things on it.
Also, if you can find it free, I suggest you the Trend Micro security suite with antivirus and firewall, both my banks gives it free to the customers so maybe you could find some institution that has an agreement with them too. It's very easy to use but on the "heavy" side of performance degradation