You have a virus. It happened to me a while ago, but I had a recovery drive, with all my files backed up, so I wasn't worried.
Basically, it slows down your computer drastically, you can't do anything, without the computer freezing. Your virus scanner won't detect it, your bust.You have 2 options:
1.System restore
2.Back up all your documents, then format hard drive, and install windows (make sure you do full format, and not quick)