Avast 5.0 is far from a resource hog. It's a very good free antivirus software. Microsoft Security Essentials, and Avira are also good options.
As suggested, Malwarebytes is a good on-demand scanner. It's very useful if you want to scan for an infection.
Winpatrol is also a good piece of free software. It has many useful tools, but one nice feature, is it'll notify you if something is trying to alter your system, and ask you to allow or deny access. It's a nice fail safe in case something does slip by the AV.http://www.winpatrol.com/