For years, people have been predicting the death of Microsoft's Windows dominance. And in the last two years, it's finally happened. The rise of iOS and Android have made Microsoft's operating system significantly less important.
Luckily for Microsoft, this hasn't meant the death of its business overall. Thanks to the strength of its Office franchise and its Servers and Tools business, Microsoft is still very healthy.
But, there's no escaping that Windows is what drives the whole company. CEOSteve Ballmer calls Windows, "the heart and soul of Microsoft from WindowsPCs to Windows Servers to Windows Phones and Windows Azure." And that heart is beating a little bit more weakly today than it was in say, 2005.
This chart from Asymco earlier this year illustrates the decline of Microsoft's Windows monopoly as Apple has risen. There are other illustrations of the same thing here and here.