Easily the most functional and helpful PC protection program on the net that offers a free version. The free version includes a full and quick scan, while the paid version offers a flash scan.
My Dell laptop recently encountered malware which prompted me to buy bogus virus removal software in order to get rid of it. Obviously this malware was a scam, as I could not even open my task manager. So I opened up Malwarebytes Anti-malware, let it run a full scan, and voila - I had complete control of my laptop again.
This has saved me a number of times, and I definitely recommend it.