Using Powerpoint in CATV?

I have a customer who has several televisions in a medical office who wishes to have a power-point presentation streamed to all televisions. All the televisions currently are hooked into basic cable so my idea was to take advantage of that. I know how use a TV tuner card and get a signal out of a computer so that is not a problem.

My question is how do I go about sending a simultaneous TV signal from a computer without affecting their ability to use cable TV? Can I hook it in to the coax cable signal and create my own ‘channel’? What kind of hardware/software would I need for that?