A good question about US monopolies. At various times, the US government has stepped in to breakup actual or virtual monopolies; the last major one I can recall was the breakup of the original AT&T, who controlled virtually all of the telecommunication in the US. The result was a number of smaller, regional phone and communication companies; however, over time, Texas-based Southern Bell Co. went after acquiring some of the other regionals and reformed itself as...AT&T. The new AT&T does have some competition (Verizon, Sprint, etc.), but it now controls a huge percentage of the telecom market...
As fa as Microsoft goes, there have been rumblings in the past on the subject of breaking up MS into smaller entities; MS, itself, had, at times mulled over splitting off some its operations, but nothing has come of any of the musings. MS actually does have a good amount of viable competition in the various markets in which it trades, so a government ordered breakup is also not likely anytime soon...
I recall about 25 or so years ago there was an article in a business publication debating the idea of breaking up MS; one of the experts cited in the article made an observation based on the then notoriously cutthroat and aggressive marketing strategies of MS; he asked if breaking up MS wouldn't be tantamount to breaking up a vicious giant killer whale into swarms of killer piranhas...
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