I used Norton for many years until I started getting problems with OCD. I changed to Microsoft Security Essentials and solved all my problems. MSE is one of the best antivirus and is free.

MSE has great detection rates, particularly for rootkits. Even more impressive is that MSE has very few false positives, is light on resources and is good at removal of existing malware.

MSE is a great choice for average users because of the minimal user interaction required. It automatically updates and removes threats. No registration is needed, apart from a quick validating of Windows, and there are no nag screens or advertisements.