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No, Microsoft's monopoly is in no danger whatsoever. To the extent an extremely large and confused organization has emotions at all, it should be embarrassment that they shipped Vista years late, features missing, and of dubious quality. But I don't think they feel that either. What they feel is secure that no one is threatening their monopoly for the desktop in any way.
The Mac has their market share, and don't seem poised to get much more. They're just not competitive (price means something in the real world). Linux fans thought they had a chance at unseating MSFT for the desktop, but that was a pipedream (it did succeed in pretty much destroying Sun for the back end, though). Dell sells Ubuntu-equipped machines directly, but that's more a sop to the Linux true believers than a credible market play.
MSFT are so secure in their monopoly that they can deliberately bloat up Vista so you need to buy a new machine in order to run it. They and computer manufacturers know that they have a real problem in that all the current PCs run just fine and everybody's happy with XP (finally). Who needs a new PC?
All this is just to show that the image of MSFT looking nervously over their shoulder at Macs poised to take over the world is truly Koolaid. Apple is just not a significant enough portion of the market to concern them. In fact, MSFT put money into Apple when it got dicey just to keep the Treasury Dept. at bay. The government likes to see competitors, and if you have to invent them or prop them up, so much the better.