How do I back-up the registry?

A simple way to backup the registry without third-party tools is this: open
Regedit, and on the computer icon, right click and hit export. Make sure
you’re using the icon for the computer, and not one of the 5 keys, otherwise
you won’t export all the keys. This creates a .reg file. Simply clicking on it
in explorer will import it back. This will cause it to MERGE with the
current registry, so any new keys created after the backup will not be
affected. There are other ways, many other ways, to back up the registry,
like a system state backup using the built in windows backup tool. But
exporting can be the simplest.
You can use the export to export any key. Make a neat change to the
registry and want to share it? Right click the key, and export just that key.
The .reg files are nothing more then text files, and can be edited with ease.
If you have any sort of basic programming skills, you can make a program
that makes .reg files to change registry settings.