I already wrote "ERUNT" is a free, very easy and fast way to backup fully your registry.
Website = http://www.larshederer.homepage.t-online.de/erunt/
It provides also an easy and fast way to restore the backups.
I have a long and successful experience with "ERUNT".
As far as I'm concerned the build-in possibility from MS Windows '"regedit" is not recommended to backup and restore the registry.
It's true you can download also an excellent registry optimizer (compactor) called NTREGOP from the website of ERUNT.