Littleern, "System Restore" is by far the easiest potential way to fix your problem. It's a lot less faff and a lot less risky that manually editing the Registry.
Start your computer and immediately start tapping the F8 key quickly until you arrive at the "Advanced Boot Options" screen. At the top of the list you'll hopefully have a "Repair your computer" option so select it and follow the prompts. At the final screen choose "System Restore"
(see here) from the list and choose an available restore point that immediately pre-dates the start of your problem.