Xfinity and Centurylink offer Norton 360 as part of their packages.
To remove Norton, use the Norton Removal Tool.
Qwest/Centurylink - Use the Centurylink Installer
Comcast/Xfinity - Follow the directions at the bottom of this page
Avast! - Boot time scanner works pretty good
Microsoft Security Essentials - decent, fairly low resource usage
TinyWall - Low resource no popup firewall - manually add exceptions or use learn mode
Malwarebytes - Malware detection and removal
SuperAntiSpyware - Malware detection and removal
RKill - Kill apps that prevent you from running a scanner
ComboFix - small and effective against malware, read the readme before using
ADWCleaner - remove Toolbars and Browser Hijackers
D7 - Experts only
PC Screen Watcher - Monitor computer use with screen shots and time limits.