You can feed your cable box out to something like the Hauppauge Colossus. I don't know anything about it, but if your cable box supports component out, you can feed that to the Colossus and record it that way.
Does your cable provider *require* that you use a cable box of some kind to get the channels you want to record? If not, you could use a tuner card of some kind and record straight to a PC. Hauppauge makes models that can tune either analog cable or clear QAM digital cable.
For example, my cableco puts its standard lineup--channels 2 through 93--out in clear QAM, so that I can plug the cable straight into my TV just like before when it was analog. Those channels are overall not HD, but the cableco does put the local over the air digital feeds--HD, subchannels, etc.--out over the same clear QAM. That means even without a cable box, the appropriate Hauppauge tuner card would tune and record those local HD channels.