I use Avast. It's never given me problems and I don't really notice a lag in performance when it scans unless I'm in like, Adobe Illustrator or something.
Ahh, Norton. Stupidest thing that could ever come with a computer.
"Internet Explorer is trying to access the internet." -Norton's spammy alerts
EDIT: My dad had/has Zone Alarm. All I remember about it was how it was more liberal with its alerts than Norton. In fact, now that I think of it, that above quote may have been from Zone Alarm, not Norton. It's been a few years so I may have them confused.