I'm assuming what you want to do is watch TV on your computer. If this is the case, you will need, at the minimum, a Video Capture Card. With a connection from the Cable Box to the capture card you can view (and record) TV on your computer using the cable box as the channel changer.
Another option would be to get a TV Tuner card. Take the the cable directly from the wall socket to the input on the TV Tuner Card and you can now watch TV on your computer without the need of a cable box (see note). The TV Tuner card can change channels.
Note: Cable companies scramble some (if not most) of the channels they provide. A TV Tuner card can not unscramble these channels and you will not see them. The only channels you will see without using a cable box is those channels sent to you unscrambled. What channels those are is completely up to the cable company and your location.
-Wolf sends