> I don't know, but they are competitors in one huge market: desktop computers (sort of) and operating systems.

Kind of, but not really. Apple gives its operating system away for free, and sells hardware. Microsoft sells its operating system to OEMs and business customers. They aren't directly competing in the way that Coke and Pepsi are.

And Windows (of some version) still has >90% market share for desktop OSes, so OSX is not a credible threat to Microsoft in that space. Microsoft's biggest obstacle in the desktop OS space is Windows XP.