Ok, if the government doesn't interfere then RIGHT NOW, standard oil would be the world's only supplier of oil. THINK ABOUT IT. they, as a company will do what it takes to control market share (and they can because they would have the resources to do so). They would just lower prices until everyone is out of business and jack them up when they are all alone.
First of all, read nate's post here. Second, these kind of monopolies can indeed come about occasionally in the free market, and they are actually the ONLY type that I know of that do, but they do not last forever unless they're propped up by the government. People are not truly so stupid that they would repeatedly fall for the price-lowering trick. Furthermore, from what I remember, Rockefeller used tactics that would be illegal under common "don't kill your neighbor law." I could've sworn I read something about sabotaging competitors, as well as bomb threats or something of that nature...
In terms of Microsoft: Microsoft built their near-monopoly on
copyright laws, which are in fact government constructs, not elements of a free market. If you're the government, do you want to know the easiest way to take Microsoft down? Just stop forbidding everyone else to copy Microsoft software.

Also, operating systems and similar "framework" products that other products are built on top of are in a unique position in the market, and the dominant company tends to be much harder to displace than in other market sectors. That said, I use Linux anyway (usually), and it's a perfectly viable alternative.
As a matter of fact, corporate personhood in and of itself is a government construct...and one which should not exist.
I know a lot of us here use a very exasperated tone when we discuss this stuff...and it's mostly because we've written essay after essay on it already, and we're just tired of repeating ourselves when new posters could easily read hundreds of pages of past discussions before bringing the same tired arguments to the table. For instance, here are two of my longer essays on economic topics:
http://www.ronpaulforums.com/showpost.php?p=1630316&postcount=23
http://www.ronpaulforums.com/showpost.php?p=1633111&postcount=47