I use Microsoft Security Essentials. I haven't used Norton in 4 or 5 years, but I quit using it because it seemed to be a huge system hog. MSE seems to work well, I've never had anything I knew about that it wouldn't catch.
For added security, use Adblock plus and NoScript on your browser (I use FF with these). NoScript is rather annoying, but if you use it you can surf in relative safety, as long as you don't open the door for a shady site. I'm pretty sure I haven't had any malware since I started using No Script.