Any PC would do if it has:
- a TV-card to connect it to your cable or whatever way your signals arrive
- a video card with TV-out possibility.
- the right software (but you're not asking about that)
- a not too small hard disk
You might have some requirements about the quality (analog SD or 1020p HD definition) for both incoming and outgoing connections, but you don't specify them.
In stead of building your own mediacenter, consider buying a hard disk recorder. Much easier, and probably much cheaper also.
I don't know if this is the right place for this question but here goes. I'm thinking of building a computer to use as a harddrive to put movies, tv shows on. So i can watch them on the tv. What components do I need and what are the best ones to get for this?