OK, normally I don't respond to tech boards but I just figured this one out after struggling with it for about 3 days after reinstalling windows on my machine.
I have a GeForce FX 5200 card - NVIDIA software.
Here is what you do - to go control panel - display - Advanced - then the GeForce FX 5200 tab - go down to Full screen video - in that tab you will see Full Screen device - make sure that it is set to Secondary display. Then hit apply and OK.
Now you can play your video on your TV while at the same time be able to do what ever you want on that computer and it doesn't show up on the TV screen.
In addition, if you want to play youtube clips or games on your TV you can.
The TV can now play VLC player videos and when you aren't playing the videos it clones your computer monitor.
Hope that helps - I am so happy I got this under control!!