I would just do a clean install of your operating system to fix this problem. You can do this without logging in by changing your boot order to boot from either:
-a usb stick with OS installation files (fastest install, typically you make these yourself)
-a cd with OS installation files. (Not as fast, but you probably have one from when you bought the pc)
There are guides to do this if you don't know how to do reinstall windows. Just google "clean install of windows" and you should find a lot of resources!