Heya,
A decent TV tuner. The Hauppage 1250 is like $50 and has great quality. The big brother version has two inputs so that you can do even more DVR (like record one, while you watch another channel, etc) but it's almost three times as expensive. It's PCIe by the way.
A fat HDD, like a 1TB or 1.5TB to begin.
That's pretty much it. Besides software. Window Media Center can do it decently. Otherwise, there's sourceforge stuff if you want free. Or if you're willing to pay, there's other options (though I've not seen one worth paying for that a sourceforge option didn't already equal for free).
Very best,