Go to regedit. In Windows Vista or 7, run as administrator by right clicking regedit. The easiest way to find regedit is to go to the start menu and type regedit. Start deleting values or changing values. WARNING: Only attempt on a virtual machine because the Operating System will have to be re-installed.
This is the screen that will pop up after deleting registry files. Even if you pick last known good configuration, the computer will not start. The operating system must be re-installed.