I
think you should be able to do it going via the STB.
Have a read of this thread:
http://www.cableforum.co.uk/board/showthread.php?t=5389
Similar thing, except you have your STB connected to a TV card in your PC, rather than connected to an actual TV. Also, I think you'd need the Advanced SCART Lead for the XBox, rather than using the normal lead which comes with it.