I've used AVG Free and MSE. Not sure which one I would be happier with as they both perform equally well. Never noticed any resorce hogging from either of the two since I leave my PC on 24/7, and have complete scans ran very early in the morning when I'm not up.
What I currently run:
MSE
Windows Firewall
SuperAntiSpyware
CCleaner
Not being able to get to an infected site to begin with is the majority of the battle and
amazing defense.
OpenDNS
Custom Host File