I believe in the 'watchdog' theory- I run 4 different things
adaware, spybot s&d, NAV, and spyware blaster. Usually 1 will miss something, but the others will pick it up. I don't particularly trust norton, but I'm cheap and can't afford a decent firewall (I hate zonealarm), and I keep backups of my registry via norton and spybot (not counting the ghost backup). I once bought a used PC with norton firewall 2002 that was expired and it worked just fine online so I dunno what they did to make the newer versions restrict access when the product expired, but I DO regularly review the firewall rules just to make sure norton isn't allowing something I don't know about.
I've got an old copy of Mcafee somewhere, but I'm sure they operate along the same lines when it comes to subscriptions.