Not to say MS isn't being dumb about this but the guy could technically be right (probably something in the EULA as mentioned before). There are conditions on being able to buy an OEM version of software, usually a harddrive or motherboard purchase. That may bind you to use the OS on that specific hardware and since you changed out the motherboard it voided the "contract". Just my thoughts on why the guy told you he couldn't give you a key. However, I agree with everyone else, call back in and talk to someone else.