* Your statement about a monopoly being defined as an entity that lacks economic competition and then clarifying that owning 90% of a market qualifies is an obvious contradiction.
* Legally speaking, you are correct that the government defined Microsoft as a monopoly. The government can use laws and edicts to define "monopoly" in any arbitrary way that it wants to. If the government declared that 80% ownership or even 51% ownership of a market was a monopoly, legally they would consider themselves to be correct. But this fact does not necessarily imply that their definition is objectively correct and based on a proper understanding of economics.
Considering this is an absolutely huge global market, which includes countries that have an aversion towards anything made in U.S. - and considering that more than 90% global market share really means 100% in local markets, Microsoft's Windows monopoly has been one of the biggest and most powerful in human history, being actually a textbook example in economics courses. Many other historical monopolies were only local.
It's also a well known fact that Bill Gates invested in Apple in 1997 and ported MS Office to Mac OS to keep their main competitor (that was also largely irrelevant in 1997) from dying. Bill Gates is one hell of a strategist and this happened before the anti-trust case being brought against them. Bill Gates foresaw that with Apple dead, they'd be in one hell of a rough spot.
Don't weasel your way out of a broken definition by hiding yourself behind phrases such as "legally speaking". In the making of those laws the legislators do have good input from economics researchers. Sometimes laws are inaccurate, but they sure as hell beat an individual's personal feelings.
* Legally speaking, you are correct that the government defined Microsoft as a monopoly. The government can use laws and edicts to define "monopoly" in any arbitrary way that it wants to. If the government declared that 80% ownership or even 51% ownership of a market was a monopoly, legally they would consider themselves to be correct. But this fact does not necessarily imply that their definition is objectively correct and based on a proper understanding of economics.