[nathanb]

I'm assuming that the computer is running Windows and that you don't have something useful like an sshd, telnetd, or VNC client running on it. The IP address is only useful if you can connect to the computer in some way.

The one thing you know is that the mail reader is still running. Working on the assumption that it can't hurt, maybe craft a mail to the account to either fool the thief into installing software like Prey discussed on other comments here or to fool the thief into telling you where he or she is.

For the first one, just send out a mail saying something like "Click this link to install HOT VIRTUAL PORN". Make it look like the average spam mail (though not spammy enough to get filtered). Obviously you would need to repackage the Prey installer as some sort of porn software installer, which may be easy or hard depending on how silent the original installer is.

For the second one, post a Craigslist or equivalent ad offering to buy a laptop computer like your friend's. Then send a mail to your friend's account as yourself saying "hey, I saw this ad and I know you're looking to unload that old laptop of yours, so thought you would be interested".

If people fall for Nigerian 419 scammers, maybe your thief will fall for this. Assuming he's reading your e-mail.