Hi
I have both my HD TV and my monitor hooked up.
I got to it by going to Control Panel, Display, Change Display Settings.
Once there you should see options for how you want to treat the displays attached to your computer.
If you don't see both of your displays hit the Detect button, if the display is connected through you video card it should show up here.
There are options for displaying on either or both displays.
Under the Multiple Displays drop down you can opt to extend the display across both, duplicate the display etc.
Mike