At 90% of the personal computer operating system market, they are still considered a monopoly... and as such are constrained from certain practices that competitors such as Apple are not obliged to eschew. I grant you that their monopoly is fraying around the edges faster and faster... and even has some gaping holes appearing in the weave...
I disagree -- there are too many credible alternatives these days to complain that Microsoft can force people to buy their products.
Go back to the mid-1990's and the picture was very different -- the Mac was ineffectual, Linux didn't exist as a product. If you wanted to get things done in the mainstream, you pretty much HAD to have Windows, and of story.
The rise of Mac OS-X changed that for consumers, and the rise of Linux changed that for more technical and more experimental users. It won't go back.
Microsoft's position has been eroding quickly since about 2003, and they are now in the position of the Empire in Asimov's Foundation -- Trantor still shines brightly, but the structure is weakened and visibly crumbling.
I believe that we will see Microsoft drop to 70% marketshare within 5 years. The only reason that can happen is that they are no longer a monopoly.
There's monopoly, then there's abuse of monopoly. When your marketshare is that big you have to be more careful than everyone else not to abuse your monopoly, but you can have a far lower marketshare than that, less than 50%, and still get nailed for monopolistic practices.
Monopolies can do good or bad. Standard Oil brought efficient production and use of materials, providing cheap oil to the masses. It deserved to grow big. The main people who lost out were the less efficient competition in the early days, but even then serious competition was already forming before the breakup in the form of Shell, Gulf Oil and others. IMHO, pure market forces would have killed the Standard Oil monopoly by the 1930s.