Well, it looks like Zelig has covered this pretty well, but I'd like to add a few other arguments who Microsoft isn't going to implode anytime soon.
1) Microsoft Windows had the monopoly on PC-gaming with things such as DirectX. That seems about to change: Apple gaming is increasingly accepted, and soon Valve will launch a Linux version of Steam. In this pace, Microsoft Windows won't be alone on the PC-gaming market soon. In fact, it already isn't.
But almost anyone who puts PC gaming as one of their factors when buying a computer is going to buy Windows. It's cheaper, which gives you more money for the actual games. It's where you can get the best hardware (especially graphics hardware). You can count on any PC game you want to play being on Windows - a lot either aren't on Mac, or are delayed on Mac (and a considerable amount of PC games being available on Linux?
).
And then there's the console market. Which these days, you pretty much get either a PS3, or, increasingly often lately, an XBox 360. Which happens to be made by Microsoft. So I think Microsoft's doing OK in the gaming realm.
2) Casual users find Apple increasingly appealing.
Perhaps, but not all casual users can afford the Apple tax. And even among the ones who can, some of them have other priorities on what to spend their money on, and just want the cheapest computer that'll let them surf the Internet or whatever. Even if that whatever requires a $1500 computer, they might find that a more appealing option than spending $2000 for the same machine made by Apple. Money still doesn't grow on trees.
Even if it's not super-popular, neither were Windows ME or Windows Vista, and the latter was Microsoft's latest OS for more than 2.5 years. I'm sure if the reception isn't the greatest, Microsoft will let Windows 7 continue to be sold for awhile just like they did with XP, and then Windows 9 will come out, having addressed the main concerns, and everyone will love it just like they did with Windows 7 (even if Windows 9 is basically Windows 8 with polish - after all, Windows 7 is basically Windows Vista with polish).