This article is a excellent opinion post on Microsoft's current and future status as a monopoly.
Microsoft Windows still dominates the desktop. But in many other areas, including Web servers and supercomputing, Microsoft is just one player among many, and often a weak player at that.
But no matter what Microsoft does, it will never have a software monopoly again. Nor will any other company. The barriers to entry in the software business have become too low for that to happen, and too many skilled software developers are learning that they can earn at least as much working for themselves as they would by working for big software companies.