Technically speaking, they don't have a monopoly. If people don't like the OS, they can always go Mac or Linux.
That said, I think their problems are their own fault. Because Windows is so indispensible to most people, M$ can charge what they like. They have every right to do this. HOWEVER, due to this ability, they are charging way too much for their product, which has spurred people to pirate it, creating a huge legal and financial fight. The issue here then isn't so much a monopoly in the sense of cornering an entire market (which they haven't), it is that they have created a market which they, of course, own.
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