Answer:
Microsoft can be considered a monopoly (although it is not by strict definition), since its Windows product is so popular that Microsoft is able to set its price at will. It is not really about market share, but about market power. For example, Google is also HUGE but it has competition.
In my opinion, the real problem with Microsoft is that is has used its market power to halter competition. Who will invest in a software company that will try to compete with Windows, if only PC's with Windows installed are sold? The answer is to sell different computers.
Finally after many years competition arrived, but in a different type of computer devices: smartphones and tablets. Smartphones may technically be considered telephones, but they are very powerful mini computers. Before the iPhone existed (and later Android devices), Microsoft had no real competition. Now PC sales are constantly decreasing while everyone has at least one smartphone and/or tablet (many have both).