I heartily agree with JohninKansas about keeping your virus program up to date. Otherwise it's like having an empty fire extinguisher hanging on your wall.

Two programs I've been pleased with in addition to the antivirus and firewall:

WebWasher to prevent popups (much better than PopUp Killer, in my opinion). The only problem I had was that it kept hanging up when I tried to download an update. I had to remove and reinstall the program. Haven't tried it again. But the program prevents at least 95% of popups from showing up on my screen (the icon in the tray changes from white to blue every time it blocks popups), and doesn't seem to interfere with other sites.

AdAware to remove all those spyware programs and data miners that insinuate themselves into your system and tell their masters all about you and your web surfing and buying habits. Their site is one of those that is designed for IE and looks a little funky on Netscape...which I don't like in principle, but the program seems to work well.

Aloha,
Mark