I see Malwarebytes has been mentioned, that is excellent and I use it regularly. For a free one-off scan, go to this link
http://live.sunbeltsoftware.com/ and follow the instructions. This progam is basically a scan once and forget it, so if you want to use it again you will need to download it again. In otherwords it does not reside on your computer like a normal virus checker.
I use the company's main program Vipre (not cheap, but good) all of the time.
The downloaded program runs a deep scan from within dos and can take several hours to run, best do it overnight. I always use it on customers computers if they are having problems.