For antitrust purposes, a monopoly doesn't mean what most people think it means. Antitrust is about behavior, not about size.
As my antitrust professor used to say, owning the only movie theater in town may be a monopoly by the dictionary definition, but it isn't an antitrust violation. Being one of two movie theaters, then buying the other one and closing it down, is.
There have been reported antitrust cases where a company actually controlled only 5-10% of the market-- but used that 5-10% to fix prices, block access to the market for new companies, or attain control of a supply chain to eliminate competitors. (I've often wondered whether Apple could get into serious antitrust trouble for supplying Apple Stores with Hot New Items faster than they do independent Apple dealers) Recall that the big issue in the DR-DOS suit involved Microsoft modifying Windows to detect DR-DOS and then not run.
Microsoft may not have the market share that it had a few years ago in certain areas. But they've definitely continued some of the behaviors that got them into trouble in the first place-- with file formats, with chat protocols, etc. It happens that the Bushies don't take antitrust enforcement very seriously, so it's been largely ignored in this country. But that doesn't mean that the potential isn't there, especially if the abuses get too egregious, for them to get whacked a whole lot harder than they were the last time.