Basically; if you're not looking to buy a Windows XP Media Edition PC then you will need to buy a WinTV Card. Or another TV Viewing Card.
That will take your aerial connection into it and play all of you TV channels on it. A big one will come complete with great Videoing abilities - there is a specific one which does this but I cannot remmember the name. ask in a PC World for WinTV and they will show you. Basically its a PCI Card which you put in, installs and shows TV.
That will pick up Sky; and although you will have to change Sky Channels throught the Box you will be able to record the programs (not encoded things like Box Office).
You'll also need a Nice Big High-Definition Monitor or LCD TV to use a Computer properly as your Media Centre.
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.