how do I hook up my digital cable box through my computer?

Through windows media center I have always watched regular cable tv by hooking up the cable wire from the wall to the back of my computer. Now I just got digital cable using a digital box and I would like to know if it is possible to hook up the digital cable through the box into my computer? There is no cable out hook-up on the back of the digital box, only hdmi and regular red white and yellow rca hook-ups. I have tried the hdmi to the hdmi on the back of the computer, and I also tried the rca's to the "composite video" hook-up on the front of my computer but still no signal. Can someone please help?

Thank You in advance

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