For mobile computing Apple has definitely position Microsoft against itself, classic "innovator's dilemma" style - Microsoft has extracted huge profits from their intertwined cash cows from enterprise businesses (who pay today for the licensing of promised future revisions) - highly lucrative, but binding Microsoft to produce those new versions that run business critical desktop software.
Along comes Apple offering something stylish that consumers want (and are now purchasing in vast numbers), but which will never run "desktop software", as the entire paradigm of mouse+keyboard is thrown on it's edge.
Microsoft is caught in a bind - one one hand, if they ignore the new market, their old one will be dwarfed and their "network effect" will dwindle, draining their monopoly pricing.
On the other hand, they can't jump with both feet into this brave new market because of their pre-existing contracts to enterprise (they could develop two separate versions but that would again, cause them to lose more profits due to the increased operating costs).
And so you have Windows 8, which tries to be both to two separate markets, and yet, neither - at the same time. Highly ambitious considering all of Microsoft previous failed attempts to address tablet and mobile computing.