As I understand, Microsoft's monopoly depends on "lock-in" strategies. <http://en.wikipedia.org/wiki/Criticism_of_Microsoft#Government_anti-trust_suits>
It's not uncommon; many software companies use them. Once they achieve a certain amount of marketshare, they may be able to use "network effects" to lock people in.
For example, suppose we're in some pure capitalist world, and your phone company won the most marketshare. You'd probably lock out my customers' phones from your network, or raise the price for them or something. You can do this because your phone company is much more attractive than mine, since you have more users to call.
In the case of Windows, it's a "platform" whose strength comes from all the decades of software written especially for it, which many people use. (There are ways to attack this lock-in, like ways to make MacOS or Linux pretend it's really Windows.)
Also, Microsoft probably still signs secret contracts with computer providers like Dell. Giving them preferential pricing if they preload Windows on all machines, as well as maybe punishments for loading competitors. I'm not sure if this is legal, but...
Tayssir