After reading that section of your text, consider if Microsoft make-ups such a large proportion of the personal computer operating systems market that the market should be considered no longer competitive. As of mid 2015, approximately 86% of personal computer operating systems were sold by Microsoft (Vista and multiple versions of Windows). Per your books definition of a monopolist, the existence of other operating systems make Microsoft not technically a monopolist. However, this exceptionally large share of sales in the personal computer operating system market clearly establishes Microsoft as the dominant firm in that market.
In your initial post, discuss why you think that Microsoft's current share of sales in the personal computer operating systems market is (or is not) large enough for Microsoft to set prices for operating system software as a monopolist? Also, how big of a share do you think a firm needs to have of a market before it can set prices as if it was a monopolist? Lastly, what do you think the makers of other personal computer operating systems could do to gain a larger share of sales in that market or cause Microsoft to lower the price of the personal computer operating systems that it sells?