Run the Norton Removal tool to remove any crap left behind.
http://www.softpedia.com/get/Tweak/Uninstallers/Norton-Removal-Tool.shtml
For free Anti virus id recommend Avast over MSE.
to Paul: remove Norton; remove Microsoft Essentials...then wait til Microsoft removes; then go to Microsoft Security essentials and let them download again and see if your new version works. I've had one fail with some error...then removed se and went back for a fresh download and it worked. - icarususer.
What exactly happened?couldn't get it to work