SuperAntiSpyware Free Edition
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Full user review
It's dependable and underrated. It can find and deal with extremely nasty viruses.
I don't consider it a con to have to update an anti virus program which I haven't had to pay for.
My first use of SAS was as a last resort after a free online scan in safe mode by a top anti virus application failed to get rid of my virus. I was contemplating reformatting my computer. Superantispyware found and dealt with the virus and after rebooting it was gone.