I use Firefox (latest version) and Ad Blocker Plus. In addition I have the free AVG antivirus, Spybot Search and Destroy, and Free Windows Registry Repair software. My Windows firewall is on. I run Spybot and the registry repair a couple times a month, checking for updates in each of course. You have to run the registry repaid about three times to take care of everything possible, but it doesn't take long to run.

I've done some tweaking of the firewall, but I don't remember exactly what I did and I'm not sure it matters.

Oh, yeah -- I'm on XP.