This is how you do it.
Backing up part of the registry
1. Click on “Start” then select “Run”
2. Type “regedit” (without quotes), and click “OK”. This will start windows “Registry editor”
3. Navigate to part of the registry where you will make changes
4. Right click on key and select “Export”
5. Chose destination folder and file name and click “Save”
6. Backup file with extension “.reg” is saved
7. When you need to undo changes, just click on backup reg file and original values will be written back into registry.
Downside of this method is that will not remove keys or values that you added, it will only reset changed existing values to their original values.