Not really.
Which corporation do you believe is a monopoly, and why?
We can stay on topic with Microsoft: excessive market share / lack of any real competition.
I believe your assessment is prejudiced by your own misconceptions about the market.
No, MicroSoft truly doesn't have any real competition.
Lack of competition doesn't constitute a monopoly. Exclusive ownership and control are necessary to have a monopoly.
Is anyone prevented from competing with Microsoft?
Are there barriers to entry into the market? For example, is there anything preventing somebody from developing an OS of their own to compete with Microsoft?
MicroSoft truly doesn't have any real competition.
You're quite mistaken. Linux is Microsoft's worst nightmare. Linux is taking huge bites out of Microsoft's market share, both on the desktop and in the server markets. Apache runs over half the world's web servers.
If Microsoft had no real competition we'd all still be running MS-DOS.