How to backup/save/export a registry entery in windows xp,vista and 7

1. Click on "file" on Menu bar.
2.Now click on "Export"

3. it will open the browse window.

4. Now open the desired location where you want to save the registry file and give a name to that registry.
5. that is it. Now open that location you will find a back up of your registry. (i.e. the file with .reg extension)
Must check:How to make back up Only a single Registry key or DWORD value?