Well, at the very least, do you have all of your e-mails with him saved? If so, it wouldn't be very hard to prove that you've been saying it's only Windows 200 all along. It would be his story that has changed. He doesn't have any proof you said it included Office 2000, you have proof saying it was only Win2k.
And DON'T send him Win2K out of the blue. If he agrees to stop harassing you in exchange for sending Win2k back to him, then send it back. Obviously if he sent it back he doesn't just want Win2k, sending it back to him might not shut him up. If that's the case then he'll be harassing you still AND he'll have your copy of win2k.