I've never had a problem running McA, but have recently switched all my machinery to Norton simply because it's impossible to "penetrate" the McAfee home page without picking up "13 Tons" of clickies, popups, JavaJunk, and other trash.
Email and web surfing are the two places where one most commonly encouters virus problems. I don't open email from unknown senders (or unknown attachments from friends), and I avoid web sites that are full of trash. (I'm reluctant to go to .pbs, and wouldn't touch anything with "Disney" in the name. It takes too long to clean up after you've been there.)
If you have enough garbage to interfere with your antivirus program, it's probably affecting everything on the machine, and you need to get regular about deleting "temporary internet files," all cookies (except mudcat, of course), and doing the defrag thing. It really does help.