M0T said:It would take him maybe an hour to get another hardrive in the pc, even if he knows very little about computers he'd probably still manage it in order to remove incrimination.
Equally though you would have a problem that if you removed anything then he could say you put it there to frame him.
The best bet is to not say anything about it and call the police, unless of course he knows you have found it.
He could do that if you decide to tell him, which would prove hes guilty and surely if theres all the files still on there must be emails to him or anything with a form of identification on . Any person could just buy a new pc and say that the pc which was givin to the police was not his . thinking about it the best way would be to phone the police and let them sort it out