Usually, when you wish to output the primary display of a pc to a second display, you need to "enable" it - usually a combination of pressing the function key and f5/f6. However, while this approach works with laptops, I don't really know about desktop machines.
The basic reason is I think to do with the power of the graphics adapter itself. The function key switches video to the primary display or the secondary display - but most adapters cannot display on both at the same time.
My Nvidia display adapter offers an option to auto-detect for TV displays, so could I suggest you start there? Right clicking on the desktop, and then on "properties" will bring up a dialogue box for your adapter and desktop settings.