Try going to "Start" and then in the search box type "System Restore". (Without the quotes)
Next locate the most recent restore point prior to you making this change. Click to choose that point to restore the computer to.
NOTE: This will also remove any programs, drivers, updates, etc. that have occurred since that point. It won't delete regular files, just anything that was 'installed'.
This will take a few minutes, at least. It will restart the computer (sometimes more than once) and the problem should be resolved.