I'm not strong in business, but my sense has always been that a monopoly is when one company controls a corner of the market and has no competition. I know that deregulation often puts an end to this, and competition is encouraged with the presence of other similar businesses who provide the same goods or services.
Each market structure consists of diverse characteristics; a monopoly is defined as a persistent market situation where there is only one provider of a kind of product or service.
The central idea with a monopoloy is that there is no competition whatsoever.
A firm is a monopoly if it is the sole seller of its product and if its product has no close substitutes.
Personally, I tend to think that Microsoft is not a monopoly. Its biggest competitor (I think) is Apple. Apple is not a company to be dismissed casually, but it does not have Microsoft's market share. There is a great deal of competition between the two companies. While Microsoft is a big name in the market place, Apple holds its share of not only loyal customers but goods and services that Microsoft cannot duplicate or compete with.
In terms of market share, it was reported a year ago today online that Windows' market share had dropped below ninety percent. And it is not because sales were falling, but because there are so many other "mobile operating systems" available: Apple iOS and Android... Macs do very well, able to perform operations with specialized software that Windows cannot—which impacts Microsoft.
While Microsoft may be used to dominating specific parts of the market, it does not control them. There are other companies free to operate and compete with Microsoft. And my understanding of a monopoly is that it can only exist if no one else is permitted to compete in the market place.
With that said, in 1999 and 2000, the courts ruled that Microsoft was a monopoly for PCs. The Justice Department noted that this ruling was something that would benefit consumers. When there is competition, prices must also be competitive. However, because the market share is so high, it does not offer a great deal of room for competition to get a foothold, so I expect that is why Microsoft was found to be a monopoly.