using RCA cables can just use use a double adapter so one set of RCA's goes to the tv and one set goes to the media pc? Or do I need an amplified splitter?

easy solution: put a tv tuner in your PC
easy solution#2: connect the STB to your TV through the RCA leads, connect the "monitor out" of your TV into the a/v in on your PC if your TV has a monitor out plug. I don't mean a VGA monitor plug, if your tv has it there'll be a bunch of RCA plugs labeled as monitor out.

Next question, how the hell can you be able to record video in, and not have a TV tuner card? Does your video card have video in or something?

EasyCap USB thingy - Usb capture device which has in's and no out's