This depends greatly upon what area of Microsoft’s business you are asking about.
In some areas of business, Microsoft is clearly not a monopoly. For example, in the search engine business, Microsoft’s Bing is by no means the top competitor, let alone a monopolist. The same goes for internet browsers.
However, it is possible to argue that Microsoft is a monopoly in terms of operating systems. The company has been found guilty of antitrust violations in this regard. The most compelling reason to call Microsoft a monopoly is the fact that over 95% of PCs run Windows. Even if we take Apple computers into account, Microsoft controls over 80% of the market for operating systems for personal computers of all sorts. This, it can be argued, makes it a monopoly.