Three ways to back up the registry come to mind.
1. The best way suggested earlier by Bob is to create a mirror image of the entire drive so it can be fully restored later.
2. When you create new restore point, it creates a mini backup of the registry.
3. To create a simple backup of the registry for use when editing the registry so you can revert back to a good version of the registry after making mistakes... Open "regedit.exe" from an elevated command prompt. Once that's done, click on "File" in the upper left, then "Export", then place a dot/tick in the "All" option at the bottom of the window, navigate to the directory where you want to save the file, give the file a name, such a Backupreg, or something similar, then click on "Save".
Hope this helps.
Grif