Anti-Virus software is overrated...
...none of them offer you anything close to complete protection. Good training and eye for viruses and spyware is the best protection. There's two good free AV solutions out there: AntiVir and AVG. Both work well on older computers, forget running the latest versions of Norton or McAfee on, say, a Pentium III. Your older computer will take quite a hit performance-wise. He-heh, you need a new computer just to run either Norton or McAfee. :p
Which is better: Norton's or McAfee? I have fewer problems with Norton's, but that's not saying much. I don't think I ever got over having to deploy McAfee 6.0 when it came out. What a bear. After my experience with 6, I wouldn't let McAfee near a harddrive of mine. But I've had a lot of problems with Norton's too.
I usually steer folks into the free AV's and also put them onto online AV scans, like RAV, Trendmicro and Panda. Personally, I no longer use AV software, none at all. But I don't recommend that to most folks. I cut my exposure to viruses by avoiding the use of Internet Explorer, and I flat out will not allow Outlook or Outlook Express to be used on any of my computers. Avoiding MS programs, even if you're using Windows, makes you a lot safer. Sign up for a Yahoo email acc't, or even Hotmail, and you'll NEVER have to worry about email viruses.
Just my two bits...