This has happened to me numerous times, and the first time it happened I was clueless as to what to do. I quickly found out you have to download an anti virus/spyware program to a USB drive, and run the program from there. It works. My favorite is Malwarebytes Chameleon, a free program and I think it is the best, but many other programs exist that do the same thing.
h/t Natura Naturans