This is discussed here
http://www.tvforum.co.uk/forum/viewtopic.php?t =18782
Seems you cannot.
The image on the sky+ hard disk is still encrypted and uses your Sky card to decode it while it plays.
If you move the file from the sky+ box to an external hard disk it would stil be encrypted.
You could ask on here
http://www.avforums.com/
This is where all the TV experts hang out.
One option, you could record the program off Sky+ on to a DVD (if you have a DVD recorder), then put the DVD in the computer and extract the program that way.