It's a gray area. One of the issues in the Microsoft antitrust suits was: what is the "market" in which Windows operates? Obviously, operating-system software of some sort, but which platforms count in the total? Microsoft was arguing that they make basically "an OS for computers", and so their market share should be computed after including Unix workstations, Mac desktops, maybe even Unix servers and IBM mainframes in the totals. Netscape were arguing that the relevant market was the OS market for IBM-compatible home-and-office PCs, in which Microsoft had a much higher dominance.

It's a good question and it turns out to be the most important issues for any anti-trust action. If you are really interested, I would encourage you to look up some famous cases and read the decisions in some of the cases.