Ask HN: If you are CEO of Microsoft, will you build a Microsoft branded PC?
What is stopping MS from doing so? They already have MS branded phones and tons of experience building computing hardware (XBOX 360).
Some PC manufacturers would doubtless shout "monopoly" and could credibly threaten to drag Microsoft through the courts, or even into antitrust again.
That said, I can see a niche for them in selling PCs with high quality drivers (and thus a more stable Windows). But I don't think it would be profitable enough to be worth it for them.
Also, Microsoft hasn't done a great job with the xbox 360. For example, the red ring of death problem on old models and the graphics flicker on the new slim models.