This.lithium.jelly said:Microsoft Security Essentials. It's suprisingly good, normally I wouldn't trust anything that oozed out of Redmond, but this does it's job well and doesn't put much of a load on system resources. I occasionally run Malwarebytes Anti-Malware just to check for spyware/adware/etc. Never find any, though.
I wouldn't go near McAfee if they paid me, same goes for Norton. Useless steaming piles of software that they are.