The other posters are right about the Microsofty monopoly, but what made it particularly egregious was Microsoft's anti-competitive practices using their market dominance to actively damage or destroy competitors. This included:

* Contractually barring licensees from offering competing OSes, or selling boxes without Windows installed.

* Leveraging their OS dominance to get dominance in other markets such as linking OS licensing with licenses for other software.

* Pinching competitor's technology using their advantages as platform owner and rolling it into their own products (Stac).

There was more, but those were the main ones.