Depends on how you're approaching it.
If you want to use the existing set top box then all you need is a capture board that handles the output format, i.e. 480i, 480p, 720p, 1080i, etc. You run the line out from the set top box to the capture board, and run something like dscaler as a front end.
If you want to watch digital cable on the HTPC without a set top box, you need a tuner that supports QAM deccoding. The MDP-130 is an example. I'm sure there are others. Then you need front-end software that supports that card, such as Beyond TV. This combination will get you the unencrypted digital cable channels, i.e. you still will not be able to receive premium or pay per view channels. For that you need a set top box or a cable card in areas that support it.