Malware bytes goes where your normal AV has failed. This little gem of antivir software has saved me on several occasions. Its very light, very small, and a life-saver to put it simple. Malwarebytes has unique scanning features which allow it to catch even the most elusive troublemakers on your PC, I recommend it from personal experience. The program has a free and shareware version, the free one is good enough and provides excelent remedy for a sore windows, the shareware provides active defense and scanning. I've been unable to try the shareware version, but if the freeware is any clue, it should be more than worth it.
The latest version can be found here.