The difference this time around
Microsoft has always played the numbers game, in their financial reporting, and especially their "sales" figures. I don't even come close to believing Bill's figure of 40m.
The key difference with Vista is that *mainstream* media are being critical, pointing out the problems it's causing some people, and even worse, are running stories about OSX and Linux more often. Apple are also laying the boot into Windows with their "Hello I'm a Mac" ads. Microsoft haven't really had to deal with this level of open criticism before, not in front of an audience of "normal" people anyway.
The average Joe who doesn't read techie news sites used to be totally in the dark about Microsoft's issues, but is now more likely to be aware that Vista is turning out to be a problem child, and that there are alternatives (even if they wouldn't have a clue how to use or install them). Or more likely they're comfortable with XP, which they've had many years to get used to, warts and all, and really don't *want* to change.