Here's the issue. Windows Vista is coming out soon. They are of course claiming that it will be awesome and amazing. I personally don't see all that much left in it that I'm excited about, but that's not really important.
Windows XP is, as I understand it, nearing its end of life. This means it won't be sold anymore.
This next part is a pure thought experiment. Let's imagine Windows Vista sucks - let's imagine it's the worst OS since Windows ME. Back in the Windows ME days, there were two easy solutions - you could either buy Win98, or Win2k. With Vista, you won't have that option. Vista will be the only choice.
I don't think Vista is going to be *that* bad. But if it is, MS has two choices: Admit that they fucked up and extend XP's lifetime, or lose customers who want an OS that isn't bull hockey. (I imagine they'd go to OSX or Linux, depending on the customer and their needs.)
Again, I doubt this will happen. I'm expecting, at worst, a situation like the 2k to XP jump, where some people preferred the new one, some people preferred the old one, but in the end most people would be fine on either one. (At least, once you turned off the XP theme.) And with luck Vista will be a real leap ahead - I'm divided on the "let's hope MS does well because I still use Windows for games and I'm selfish", "let's hope MS does well because competition is always a good thing", "let's hope MS does well because I don't want to wish harm to anyone", and "burn, suckers" fronts. But if I were MS, I'd be a bit worried on this front.