- You don't need Windows Media Center
- I'm not quite sure what you want: to use it as like a Tivo, or just for playback on the PC monitor.
You're going to need a TV tuner either way. It'll basically let you plug in your cable line to your PC. Now if you only want to play the TV back on your PC monitor, that's as far as you have to go. The TV tuner will come with all the software you need.
On the other hand, if you want to playback on your TV, you're going to need additional software. Read this article, I think it does a good job of explaining the software:
Install whichever program you choose, and you'll be set.