It seems that the things that allow your media center software to control your cable box are called an IR Blaster. you stick them in front of your cable box and they send the signal like a remote control to the cable box to change stations. That way your Media Center software can change the channel.
Yes i understand about cable card tuners for PCs they are expensive and you have to pay the cable company just like a set top box I do not want them and I am not sure I could get one I asked Comcast about them a while back and they said they had no idea what they were.
So I guess with a TV Tuner you can record the composite output from the cable box and use an IR Blaster adapter to let your media center change channels all day while you are out or doing something.
I am looking at them now on amazon .. i think some plug into the tuner card and some are independent of the tuner card