This seems like semantics. An example I've used before: I'd definitely say Microsoft has the monopoly on the operating system market, considering how many people use Windows in comparison to Linux. Not that there's anything wrong with that, but to say monopolies can only exist when the government forces them to exist seems like a stretch.
You really should look up the definition of monopoly.
If Microsoft released a new update tomorrow that turned all windows desktops into a flashing goatse image (which is entirely within their power), what would all those Windows users do? I know I would install the latest Ubuntu and use that instead, because I can. That is not a monopoly.
Even if a company has 100% of the market share, if it's possible for users to stop using that company and use another at will, there is no monopoly.
Your browser example is a pretty good one. IE is built into Windows. Everyone who has Windows has IE. And yet only about 40% of people in the US use it (according to this study which seems relatively legit http://royal.pingdom.com/2012/04/23/current-status-of-the-browser-wars/). Microsoft is TRYING to create a monopoly with IE by forcing it on all of their users (there was actually a lawsuit back in the day about this), but it still doesn't work when there are better products to choose from.
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_foolmo_
'Ulti is like when your parents post something on your facebook status' - Sir Cobain