I have mine connected to my PC as a second monitor. (I use an Nvidia card) with a 24"LCD as the main monitor and my Panasonic 42" Plasma as my second monitor.
Mainly use the Plasma for movies on my PC.
Depending on your setup you could also try other configurations as well.
(pretty sure you could try to connect your cable on another plug)
example another HDMI input or a component input (this is how I have mine connected)
make sure that the cable(s) are attached correctly and that you have selected the correct input on your TV control menu.
Now on your PC screen right click anywhere and on that menu look for your graphic card menu. Once there look for something like "setup multiple displays".
Click on both screens, you can select depending on your configuration if you want the screens to display the same content or if you want to be an extension of.