Most certainly the spirit of the GPL is that it's fair use to tinker with aYes they can, since you have (most likely) breached the contract! To fix a bug in Windows you will have to de-assemble their code (if you are not on their pay-role, of course) and that is explicitly forbidden. You are in no way allowed to read how Windows does things.
work to get it to work on your hardware. Is it not fair use to share that
with other licensees of the original work? Should Microsoft be able to
prevent me from distributing patches to Windows that fix bugs or add
features?