As far as I know, what the Sky box records is the transport stream coming down from the satellite, not the signal decoded into a TV signal, ot thereabouts, which is why what is stored on the hard drive is encrypted and can't be copied straight to a computer drive. It needs to be decoded by the Sky box at time of playback - which is why if, for example, you end your subscription you can no longer access the recordings on your box.
But you should be able to record out to a computer if you have a video/sound card on that computer able to take in the output from the sky box, and re-encode it into a format the computer can store and play.
However, I have a feeling you can only copy out at relatively low quality to a video recorder or such a video card – standard definition and with stereo sound, I think – and you will have to do the copy in real time, as you do with a video recorder.