If you use outlook/express or other you might need special software as emails are ussually stored in a special file. to recover them after emptying your trash would be a little difficult. You might be able to recover bits of email and read it in notepad but i think you should consult with data recovery experts or IT Forensic analists.
unfortunatly anything you do with the computer, that involves the harddisk on which the emails were stored will lessen your chance of recovering the emails to a any format. Installing recovery software now to that drive included.