Microsoft's biggest global competitors are exactly as they were on Thursday: Nokia and Sony, two companies whose core revenues don't derive from Windows and who can set global standards. (IBM has every motivation to fund Linux development, which Redmond really doesn't like at all, but IBM doesn't have the inclination to set standards elsewhere and can't dictate consensus in the OSS space). Not that it's any consolation on Sun's Blackest Friday, but if Mankind had to be represented in an Independence Day scenario, most people would rather it was Sun engineers downloading the Trojan onto the alien's computers, and not Microsoft engineers. The Register: Why Sun threw in the towel in Mankind vs. Microsoft
Increasingly, The Register makes me laugh, but they do have a few good points as well. As per the title, it looks like "mankind" lost.
The battle over desktops has long since been fought and lost. In case you didn't get the memo, most people are still going to be running Windows. But…
Notice how I specifically said "desktops". There has been lots of noise about how everyone that is anyone takes an Apple PowerBook to work (i.e. the preponderance of PowerBooks at various tech conferences, and also in the hands of senior executives). As well, large parts of the world (Europe, Japan, etc.) don't have computers at home at all -- smartphones are their computing platform, or perhaps a gaming console.
On the server side, it makes sense for Sun and Microsoft to present a more united front against Linux: the tag line may very well be "At least it's not free software".
Apple long ago resigned itself to interoperate with Microsoft, and it has reaped the rewards because of this (basically, "Our stuff works with theirs, looks cooler, and crashes less").
There really aren't going to be any other competitors to Microsoft in the near term; there will definitely be development in the home, specifically around media centers, but there aren't going to be any new players. Solutions will be built around some form of open-source software, which as a whole can be considered one "platform". Lack of consensus and bad UI are the two weaknesses here.