I do NOT like Norton. It's a system hog, and sometimes uninstalling it can actually cause problems with Windows. Any program that makes itself difficult/impossible/dangerous to remove is a loser in my book. Add to that the fact that Symantec (creators of Norton) has attempted to cheat at least 2 families I know of by telling them their AV subscription had expired and demanding that they renew more than 6 months ahead of time, and I'll never recommend Norton again.
Avast and AVG Free have both been mentioned, and they work fine for personal use. (They are technically not legal to install for corporate machines.) I am currently using AntiVir, and I like it. The annoying pop up to purchase their paid version every time it updates gets on my nerves, and I've read that it's potentially against their EULA to remove/disable it, but can't speak to that as I can't bring myself to waste the time to read the whole EULA.
At my work, I install and maintain Trend Micro Worry Free Business Security. I like Trend, though it can be a resource hog on older machines.
If this is a personal machine, and if you have free reign to install whatever you want, I recommend AVG Free. I like it over Avast only because Avast makes you do their free registration thing once a year, which can be annoying. AVG Free seems to catch anything I put myself at risk for, which isn't much.
Having said all of that, nothing beats the antivirus that is Safe Computing Practices. Start with that, add a failsafe like AVG Free, and you should be good to go.