In my personal experience, I've used Norton and McAfee. As far as virus protection I never noticed a difference in performance, but I had both Norton and McAfee istalled on the same machine (went from Norton to Mcafee). When Norton would be running a virus scan, it would use up so many resources that I'd have a hard time doing anything, viewing web pages, opening folders or files, and after the scan was complete it would take about 10 minutes for all the applications running to speed back up to normal. It was like this on another computer in the house too, mine an intel based and the other one an amd based computer. But after my subscrition was up, I intalled McAfee, and I never had those problems again. Sure, my computer slowed down noticably when McAfee was running, but not at the same slow crippling speed norton brought it down to. Now on my month old computer, I barely even notice when McAfee is running a scan.