Don't buy a tuner to record the cable box's output. This is a very kludge-y solution that's sure to have problems. This
is what you want. It allows you to watch anything you can get on your cable box, except on demand.
You'll need a windows vista, 7, or 8 computer with windows media center. There's a plugin called "digital cable advisor" or similar. Run it and it will tell you if your computer is capable of cablecard playback (apparently it's kind of intensive). You may need to install a patch for it to work. This should be run before purchasing anything.
Assuming you meet the requirements, you will need to get the cable company to install the cablecard. There will probably be a small ($3 a month for cox) fee, but you can ditch the cable box which should more than make up for that.
gives good enough instructions for setup that I don't really feel the need to repeat them here. Just read it and follow along.
You should also get a media center remote (cheap, any one should work) as well as a keyboard/mouse solution. Do not just go out and buy a wireless keyboard and mouse. You want a trackball keyboard (this one
is the best) or an application for your smartphone. Air mouse for ios is excellent. I have android, so I tend to use an application called remotedroid. If you use this, you'll want to set the application to start up with the computer, and use a solution like taskit to make it hidden.
Let me know if you have any questions. I had a cablecard tuner setup for years, so I should be able to answer anything.