I've got 3 machines now.
On my gamer I have the McAfee security suite (which I get for free from Comcast) although I usually shut everything down while gaming.
On my word proccessor/e-mail machine I have Avast (free version) for anti-virus, SpywareBlaster for active anti-spyware protection, Bazooka to detect installed spyware, and a very old version of McAfee firewall. It works well but a full system scan with Avast is dreadfully slow (an all-nighter). Everything on this machine is/was free.
On my laptop/web-surfer, I have the same set-up as my word processor but instead of Avast I have AVG (free edition). I sort of like AVG a little better than Avast as it updates and runs a scan daily in relatively short order.
Regardless of what you use, if your going to game on it and are behind a router turn this stuff off before you log on.
For the most part, if you're behind a router and don't recklessly click executable files or allow acive-x controls to be installed from questionable sites you're probably OK. For years I've had McAfee but had it totally turned off except once a month when I update it and run scans then shut it back down and I've never had a virus or spyware (well, I have had spyware/adware but it was my own fault and I know where/when I picked it up).
If your machine came with Norton just keep it and don't worry about it but again, shut it down when gaming or when installing programs. It's also probably worthwhile to turn off the auto-updater and update manually as you see fit.