had exactly the same problem. Spanners is on the right way.
I explain now to you what you have to do using the ATI card, you did not mention which one you use, but it will be very similar.
Get the DVD ready to play on the TV with everything connected as mentioned befor, than right click on your dektop and go to properties.
The display properties panel comes up and select the tab SETTINGS. click on ADVANCED.
If you have installed the correct programs and the TV is attached, you will be able to scroll through a lot of hidden settings now, go to the DISPLAY tab and select the TELEVISION....and that was it actually....
It should revert to LCD panel as soon as you disconnect the TV, also, if the TV is not detected by the program, the check box will be grayed out.
There is also an ADJUSTMENT tab where you can select the display mode, like NTSB or PAL....
that should do it...