I will disagree with this and I am an IT person. I use AVG on my laptop which I hardly ever use but the main PC in my house is running Norton 2009 and over the years has saved my butt from having to reinstall my OS probably. You just have to set it up right so it's not a hog and completely anal with everything you do but once you have it setup just right at least me personally I think it's fine and probably more up to date and effective with virus detection and removal than some of the free ones out there.
Norton isn't bad it just got a bad reputation for being so heavy and bogging down a lot of peoples pc's so I got tired of dealing with it. I use ESET NOD 32 on my laptop, I have never had any problems with it and it is very light. I also use CCleaner and SPYBOT as well everything else that Vista comes with.