AVG, much less intrusive, easier to use. Doesn't hog CPU.
I second that motion! Get the free version here:
Get AVG Anti-Virus Free
Small footprint, doesn't slow down your system, automatic virus database updates, and is free for personnal use.
Top notch product, really. I've been using it for 3 years on multiple systems, and I don't miss any of the other "paying / subscription based" software.
Try it for a while: it's free and easily uninstallable, so no worries there; you don't like it, remove it... Have you ever tried to completely remove Norton? A real pain!
Regards,
HelzBelz
Edited: Don't be fooled by the "free" option. AVG is a paying software for corporations or professional uses; that's how it can keep up (and surpass, IMO) other paying titles. As for "missing" viruses: well, I've had the worst of luck with Norton and McAfee, while AVG would detect the viruses others have looked over... Your mileage may vary, of course.