Well....
You are certain that you never told him it included Office2000.
He has NO proof that you made that claim.
To a judge... it might be easy to see where HE made the mistake.... in interpreting the Windows"2000" to be Office"2000", if he was not very computer literate.
If indeed he made a mistake in his understanding of what you were selling him, but you have emails/correspondence to him stating that you were selling him Windows2000.... (you should have a copy of your item if it was sold on egay)..... then I believe a judge would likely rule in your favor.
He may have mistakenly thought he was buying Office2000.... but that is not your fault or problem. Providing you were not misleading.... and you have some proof that you were selling him Windows2000. If you told him (and have proof to back it up)... that you were selling him Windows2000 ("Win2K"

.... I don't see where he could have a leg to stand on in court.
However.... there is the little problem(s) of him screwing with your life. How worried are you about that? Can you be expelled from your school? Keep in mind that you might have grounds for suing HIM if he slanders or defamates you!!!!
If you are worried about what he can do to make your life difficult.... then you should weigh that. You might be better off returning his money and reselling the Win2K to someone else. If he can really screw up your life (school, etc.)
Can you tell if he opened the box? If so... threaten to contact the proper authorities..... stating that software piracy is a serious crime.
Aledge that he opened the box to make a copy of your software.
I would have him ask his attorney to contact you in writing... as mentioned above.
This will either call his bluff.... or get him to send you a fraudulent letter (which can be used against him).
JMO.