Why I don't use McAfee virus software:

[Feuer Frei!]

I use Avast Free, doesn't use much resources, 4,164k's to be exact, works fine for me.
I've used all sorts of AV's, from Norton's many many years ago, Zone Alarm Pro, AVG more recently, and now Avast.
Out of all of them, Avast seems to do it's job better then the ones i've used, scanning is quick and easy, regular updates (automated and notified to user), good detection settings, using very little resources, i guess a few things are dependent on how well a AV works:
Your system specs, what sort of online activity you perform, other utils/tools which may conflict with each other, it all makes a big difference in the way you "allow" the AV to work for you. Or not.