I use Avira AntiVir Personal Free Antivirus. It used to be called Luke Filewalker. I think its pretty good. Its got a guard thing on, so that when you download something, like something off of limewire or something, and it has a virus on it, the constant guard catches it, and asks you what you want to do with it.
I've heard norton sucks too. This computer guy told us to download this instead, because Norton had all these bugs. But we didn't have norton anyway...
Zonealarm isn't a virus protection program, its a firewall. My brother says it slows you down, but I'm not sure why he thinks that. I think Its a pretty good firewall program though. I'm not sure why he thinks it slows you down. My dad uses Window's built in firewall now.
If you have problems, you should get anti-malware things too, like spyware search and destroy, and Adaware to detect spyware/malware.