Here's a relevant story:
A long time ago, IBM held a monopoly in large-scale corporate computers. The US Department of Justice figured this out and forced them to sign a consent decree in order to curb their anticompetitive behavior.
After about 10 years microprocessors came along, and Apple, Commodore, Compaq, Radio Shack, etc were selling tens of millions of little computers. Thanks to the consent degree, IBM couldn't stop anyone from cloning their designs and "IBM Compatible" was a big selling point. IBM promptly took these sales figures down to the DOJ and convinced them they should be released from the consent degree. It certainly helped that President Reagan didn't think much of anti-trust laws.
The first thing IBM did after the consent degree was gone was release a fabulous new line of PCs featuring a proprietary bus that required a secret licensing deal to access. It almost worked, and would have if it wasn't for that darned megalomanic Bill Gates. So IBM just snuffed out their mainframe competitors instead, and they still hold a monopoly in that segment until today.
OK, what does this have to do with today? Well Microsoft can point to those 100M iPads and Android tablets and make a reasonable argument they no longer have a "PC" monopoly. Oh, also Ballmer was working the IBM account the whole time the above story happened.