I need to record HDTV on to computer - TELL ME what to buy

I want to grab the sat-box HD output and feed it to the computer and record
HD content. I want to be able to schedule these recordings. Then I want to
play it back on the TV in High Definition (1080i ot 720p). My TV already
has a ASTC tuner I don't use since there are no OTA in my area so I really
only want to record HDTV and play it back.

How can I do this ?
What hardware do I need ?