Originally used Norton (a few years ago now), but had infrequent but irritating issues with it crashing and having to re-install. Also seemed a bit resource hungry, so I swapped to Kaspersky, with which I have been entirely happy. Easy and relatively quick installation and it has picked up and dealt with a few nasties over the years. I also like the user interface
Previously had Trusteer Rapport, but heard lots of bad reports (although I didn't notice any problems) so didn't install it on my new machine. Kaspersky offers me an on-screen keyboard and "Safe Money"...I think it's called.
Have heard almost universally less complimentary reports about McAfee, but haven't used it myself.