I have used Trend, Avast, And Windows Live One Care on my machine.
I try to stay away from Norton, because as John said, it really is a
resource hog, and quite frankly the firewall is WAY too dumb. (Common sense
isn't added into the system, where as programs like McAfee know one program
from the next and will auto accept a program depennding on it's signature.)
I tried trend, but for some reason, the firewall was so restrictive, it made
simply surfing the net difficult (Wondered about resource hogging as well)
I was thoroughly impressed with AVAST!. It works well, doesn't eat up too
much space, the web runs clean, and you can break it down by only using the
Vista direwall, which seems to work quite well.
I was impressed with One Care as well. It was simple, it wasn't restrictive,
never had any issues with it, and most of all, being made by Microsoft, I
knew it would be the most efficient program running on windows. The only
issue will be after the beta mode is done when you have to pay for it, which
if I remember is like 50 a year or something. Not saying it isn't worth it
though..
Hope this helps..