TVs broadcast computer over tv channel

I have been looking for a way to broadcast my computer display over my home tv network (through the coaxial wires throughout the house) on a specific channel. For example, I would like to go to Youtube and load up a video. Then go to my TV, turn on channel 3, and see the video playing.

I tried searching for a while, but it resulted in no useful information. I am not sure if this can be done simply by software and maybe a PCI card, or if it would be much more difficult than that. If anyone has done this before, or would have information that would be useful please let me know.

A quick note: The reason I think this is possible is because I was playing my xbox (the original) one day and put Halo on pause. I went upstairs to take a break and turned on the TV to channel 3 and I saw my game paused. This was a different TV than the one hooked up to the xbox.