I use the full Mcafee internet security suite. It's been very reliable, and quite effective. The antivirus has saced me many times from getting malicious programs before they even had a chance to initialize. The firewall is effective, and highly customizable. The Clean Sweep utility is very good for keeping harddrive from getting stuffed full of internet cache, and left over files from uninstalled programs.
After having many problems with Norton, I very strongly advise against it. It's an immense resource hog, and is very prone to crashing on Service Pack 2. The firewall is acceptable, but often forgets if you've granted access to programs (even MSIE and it's own update service :roll: ). The speed disk utility caused a fatal error on my hard drive, making it necessary to reformat my entire harddrive. It's clean sweep utility was an unknown problem until I uninstalled it, it was cleaning unneeded files during start-up, slowing my computer to a crawl. It took almost 15 minutes to boot up. Unacceptable. The only program that was worth using was the spam blocker, it worked well.
I haven't tried any of the free programs, but then again, I'm not very trusting in open source programs, remember, Kazaa was free.