Never ever use Regedit for registry backups

The "Export registry" function in Regedit is useless for making a complete backup of the registry. Neither does it export the whole registry (for example, no information from the security hive is saved), nor can the exported file be used later to replace the current registry with the old one. Instead, if you re-import the file, it is merged with the current registry without deleting anything that has been added since the export, leaving you with an absolute mess of old and new entries.

Read full details here: http://www.larshederer.homepage.t-online.de/erunt/