Just having the most popular product doesn't mean you're liable in an anti-trust case, you have to abuse the position as well (IANAL though). There are no viable competitors for a PC desktop OS for them to abuse and haven't been since the 80-90's I guess.

A bit off topic, but to me it's interesting that they couldn't extend their desktop OS dominance to the mobile world when they launched Windows Phone with a huge war chest. That must have demonstrated how huge the disjunction is between desktop and mobile use-cases in general, or the strategy should have worked (and could have re-fuelled the anti-trust aspects)