I find Norton to be a bit of a hog. I've been using the Kaspersky Internet Security Suite for a couple of years and it's pretty good, better I think than McAfee, which is also on a couple of machines here at Chateau Sinister. According to the Task Manager, Kaspersky takes about 30K of memory most of time, and accumulates only a couple of minutes of CPU time in a day. My time in the computer business convinced me the only safe attitude to computer security is paranoia, and I'm not prepared to trust so vital a function to freeware.
What I like most about Kaspersky is that it's readily configurable to be as paranoid as I am about security. It was most instructive when I first installed it to find out what bits of Windows want Internet access. A lot of things want Internet access that have no business requesting it, I've blocked them all and the system works fine. My attitude is that anything that's not required for the system to operate satisfactorily is forbidden. I don't want anything coming into or going out from this machine that I don't know about, and I'm prepared to invest a fair bit of time and effort into making that so.