Yeah, you're gonna have to use the overlay options like AxByC said. With my TI4200 you have to make the TV the Secondary display and the monitor the Primary Display. Then make sure the options are set to Overlay, and make sure the fullscreen device is set to the Secondary display.
It's sort of hard to explain how to do this without knowing which videocard you have. Hell, I imagine it'll be real fun if you don't have an NVIDIA card.
Good luck!
-Lasereth