The broken XP was bought and paid for so does that not include some right to get it fixed?
No.
There is a reason OEM are so cheap, the two main ones being :
(1) The license is tied to the machine it is supplied with. Machine dies, license dies with it. There are also limitations on what can be classed as an "upgrade" of a chassis.
(2) Microsoft places a contractual obligation on the OEM to provide the End-User with a support. So OEM users should not expect any tea and sympathy from Microsoft if you try to obtain direct support (unless of course you get out your credit card for their paid support service).