Microsoft did a lot of user and interface research. It wasn't as streamlined as Apple's, but it's incorrect to say that they didn't give it much thought.
What's with this myth that Microsoft never cared about UI/UX design? It's simply not true. They're especially not any better at it now than in the 90s and 00s. Modern designers don't put even a fraction of research into UI design than what Microsoft used to do.
Making things intuitive and easily usable was absolutely a priority of Microsoft back then, and they put a lot more thought in that than most modern "UX" design.
Making things pretty wasn't a priority for Microsoft the same way it was for Apple. But I wouldn't call that UX.
I don't have a link to OS-level considerations, but here's a series of articles on how MS Office's original ribbon came to be: https://web.archive.org/web/20080316101025/http://blogs.msdn...