If your data is backed up, quite frankly I would re-install/return to factory settings - however you revert that machine. It really isn't worth all the time you may spend trying to repair the registry by booting to a bootable CD system. If you don't have a good backup, download and build a bootable CD of one of the many available for download and image to a USB drive. You will need an image you can later mount and recover the files from. Shadow protect give a free months trial - but I recommend paying for it - then you simply recover from your last good image - about a 20 minute job! I think Acronis offer something similar.