The vast majority of the 'regulars', here in AB's Technology section, use AVG Free Edition. It's consistently scores 100% in independent tests of its efficiency and it automatically updates itself every day: http://free.grisoft.com/
As Buenchico has said, go for AVG free and Zone Alarm.
After several years of using the resource hogging Norton, I am now using AVG and Zone Alarm, and the response time of my laptop has greatly increased.I am so pleased I made the change.