You'll love this... get a TV Tuner card.
Unless you have a magic bucket that will carry TV signals to your PC from your cable box so you can pour it into the hard drive.
If your video card has a video IN, which I doubt, and if your cable box has the right outputs for it, you can use that. Really though, get a TV Tuner card for the PC.
Actually, some of the TV studios web sites often have some shows they stream for free. If you can find out what format they stream in, you can find a program to save it to your drive. Something that works like RealPlayer to save flash videos off the web.
But still, get a TV Tuner card. You'll get all the software you need to record things, and will be able to pull a real TV signal.