You don't have to hack on the registry to do that.
The display control panel (and your graphic card control panel eventually) should propose you to connect the external monitor in clone mode, use only this monitor, or extend your desktop to this monitor.
You may be able to do it with some kind of
FN+Fxx key combination on some netbooks.
The clone mode may restrict you to the native resolution of the tiniest screen, but if you connect only the monitor you should be able to use its own native resolution instead.
If not, try to install the driver for your monitor.