I agree with Malwarebytes as a great free bit of software although I would be running a number of different software to identify what the cause is.
Sometimes though a virus will lock you out of downloading or running the software so first scan I would use is Eset online scanner.
http://www.eset.com/us/online-scanner/
Then after that I would run Malwarebytes, Superanti Spyware, Adwarecleaner and whatever else you can find to clean it up.
If these programmes wont respond you may have to do all this in your PCs "safe mode".
It should be possible to clean it up without resorting to reinstalling and restore points.
If it gets too hard then register on a site like Bleeping Computer and post your problems and someone will walk you through step by step.
Thing is Win Defender is rubbish(as are Norton and mcaffee), I would always switch it off and rely on Avast, avg or whichever is your chosen anti virus but at the end of the day no AV will give total protection. It doesn't take much to click on the wrong thing when you miss that little 'x'.