Excerpt from some TechCrunch mkt cap analysis:
Sure, Apple’s OS X competes with Windows, but Apple is clearly never going to license out its OS (after a disastrous attempt to do so in the 1990s when Steve Jobs was away), so its market share can only ever be as big as people buying expensive Apple machines. Apple simply cares more about profits and controlling the high end of the market, then going directly after Windows.
And actually, if both were smart and could bury the hatchet, Microsoft and Apple might be wise to team up. Google is now clearly an enemy for both of them. (Which is a bit odd, considering that it used to be Apple and Google teaming up in a major way to take on the shared rival, Microsoft.) If Microsoft really wants to compete in search, for example, their best move may be to strike a deal with Apple to become the default on the iPhone (and iPad).