Ad-Aware Free gets a very good rating from PC Mag.....
Ad-Aware Free Internet Security 9.0

Here are quotes from the article...
"Ad-Aware Free Internet Security 9.0 is our new Editors' Choice for free antivirus.
Simple user interface hides complexity when not needed. Full scan runs quickly. New technology improves malware removal. Best malware removal score of free antivirus products.

Didn't completely clean up some of the malware it detected. Real-time protection doesn't take action until just before a threat launches.

Bottom Line
Most free antivirus utilities are good at either malware removal or malware blocking, but not both. Ad-Aware Free proved very effective in both areas, almost as effective as Ad-Aware Pro. This is a great free antivirus solution for non-commercial use."