If you know the model number of the digital cable box, it may be worth searching around to see what PC software or drivers are available, e.g. pop the model number, and the keywords 'firewire' and 'record' into Google and check out the results
Just note that the FCC are enforcing the copyright protection flag on digital TV equipment , so it may also be worth looking up news groups (e.g. Google groups) for recording issues. I am not sure if it would be the drivers or software that enforces the restrictions. If only the software makes use of the broadcast flag, there may be 3rd party software that does not check.