by using the Function key (Fn) and the F8 key. This key has a Blue symbol on it that kinda looks like a laptop and a monitor.
Fn + F8 will allow you to cycle through the settings. One of them will be laptop monitor off, external monitor on.
FYI, right click on the desktop, Properties, Screen Saver, Advanced and set the "when I close the lid" to Do nothing.
Now you can just close the lid on the laptop and the picture will stay on the monitor and the laptop will not go to sleep or shut down.
P
i have a dell with xp.pluged in a 17" flat screen.how do i turn off laptop screen.my old toshiba did it automaticly.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic