Extending Windows 7 to an external monitor works fine if you have a laptop. But there are problems if you have a desktop.
First of all, when you connect your desktop to the external monitor, until the monitor begins working, you don't have a screen to refer to. So it is a shot in the dark. When I had Windows XP I could just dis-connect my desktop monitor and then connect it to the external screen I have and it always worked. But not with Windows 7. Using Win +P is not visible once you have connected.
So this is a puzzler. Anyone know how to do ths???
I do this all the time when switching between my HDTV and my Projector. Do you have a discreet graphics card installed or are you just using on-board graphics? If you have a discreet graphics card installed, the control panel for the card should detect the secondary monitor right away. Then it's just a matter of enabling the secondary display and "swapping" the monitors.
If you just have on-board graphics, are you sure your motherboard can display dual monitors?