How to Make back up Only a single Registry key or DWORD value?

 1. Open Registry Editor.
Also check: How to  Open Registry and Group Policy editor.
2. Navigate to the key that you want to save or create a back up for that.
3. Now Right click on that key.

Also see: How to add restart explorer to context menu of desktop?

4. Now click on Export.

5. Now open the desired location where you want to save the registry file and give a name to that registry.
6. That is it. Now open that location you will find a back up of your registry. (i.e. the file with .reg extension)