I wouldn't have Norton on my system if they gave it away. I had NIS on XP for several years and it was nothing but problems and the biggest resource hog I ever had on a computer. Once you try to uninstall it, it still leaves behind a lot of files in the registry. You never really delete all aspects of having installed it.
I have McAfee on Vista because it came free for 15 months. After that, I'll probably just use the built in firewall and with a Linksys router, WinPatrol, Spybot, SpywareBlaster, and Spyware Guard, I feel pretty swell covered. BoClean was another good one but I think they went belly up not long ago.
Gibson Research has a couple of neat programs for testing your system for open ports, trojkans, etc. Scroll down about half way and you'll see "Shields Up" and "Leak Test". do both and you'll see where you're at security wise.
GRC*|*Gibson Research Corporation Home Page**