In my experience it is the PC power supply which is the issue. Try removing the PC or running it off battery to see if the ground problem is there. If it is, you can work on ways to isolate the PC rather than the cable box or cable.
If that isn't it, the make CATV ground loop isolators, but they are designed for analog CATV signals, so they don't always work for digital systems or internet signals. So, you could try one (the Jensen Transformers' version is the best I've found). I used one in my home after the F-Connector splitter which split the signal between the CATV house network and the Internet modem - I put on the splitter output for the CATV signal - it worked perfectly. I no longer have CATV, so I loaned it to a friend and it worked but messed with the quality of the switched digital channels.
Another option is wiring differently - Isolate the Cable box from the system somehow - maybe a Hum-X or AC transformer with a ground lift.
Path of next to last resort would be a star grounding strap setup where every single powered device has a chassis ground heavy copper wire feeding a central grounding block.
Last resort? Bury your own grounding rod outside, maybe two, and use very heavy copper wire (like 4 AWG) to ground everything coming into your house together, including AC, Cable, and Telephone.
