If you're comfortable working in the Registry, log in using another account and open the Run box and into it type regedit then press Enter. The Registry editor shows up and form the File menu, choose Export then take a note of the name and location of the backup you're about to create in case anything goes toes up.
Next you need to navigate much the same as you would in Windows Explorer to this Sub Key:-
In the navigation pane there wlil be several long entries starting with S-1-5 and you need carefully to check that the one you alter is yours.
When you find your own entry you should have two lines - one ending with no extension and the other having the extension .bak. Edit the one without the extension so that it now has an extension .old. Next edit the other one, removing the .bak completely and leaving the entry with no extension. This looks complicated but it isn't - all I just described merely renames the backup to take the place of the flawed file.
Leave the Registry from the File>Exit menu and restart the computer - twice. Log in your own account and all should be back to normal.