Microsoft is coasting on past momentum at this point.
And between them, Apple and the swarm of open source operating systems have nibbled away a couple of extremely key markets. Name me a field where intense concentrated utilisation of computing resources happens in a mostly Windows environment. Most of the industrial computing environments run on some form of free unix-like OS for some very good reasons. And industrial computing (whether Google, Amazon, rackspace or what have you) is the growth segment of the technology industry at this time.
Windows may own the desktop still, but the desktop is a relatively saturated segment of the market. And most of the sales are replacements, not new seats.
And please don't just dismiss me as a Microsoft hater, I know some very smart people who work there and I do not underestimate the capability or creativity that can come out of the company when it comes down to the wire. But as one of the people I know there said to me a couple of years back "Inside microsoft there are about a hundred world class software companies, most of which are working against each other."