How to install a CableCARD tuner in your DIY Media Center
http://www.engadgethd.com/2009/05/12/how-to-install-a-cablecard-tuner-in-your-diy-media-center/

It should go without saying that in order for this to work you'll need a pretty new computer running either Windows Vista Premium (or higher) or Windows 7 as well as an ATI Digital Cable Tuner (DCT). The first requirement you probably already have, but the second can be a bit more difficult. Both the internal and external version of the DCT have the same functionality with the only real difference being where it is mounted. Some prefer the external since it is easier to swap out and doesn't require opening the case to install, while others will do just about anything to eliminate yet another box with a wall wart from their setup. We will warn you if you're leaning towards the internal though, it is pretty big and puts off some heat. So to accommodate the internal version you're going to need a decent size case as well as an internal USB port and free floppy power plug. Not for everyone