Microsoft's problem is that XP is probably the best OS they've come up with so far in terms of long-term stability an usability.
Sucks to be good, eh? (NOT an anti-Mac/*nix statement.)
Yep, my observation is that XP is/was the "sweet-spot" for Windows. Stable (finally), does plenty, isn't bloated, well-understood by users.
Microsoft can't follow XP. Not that Win7 isn't good -- it is. But it ain't nearly the sweet fit-to-needs-and-resources that XP became.
Microsoft is flogging a dead horse -- the NT codebase has been pushed farther than it can be and still work. MS is in the same position Apple was in the late 90's -- their operating system is a dead end.
If MS has half a brain, they're doing what Apple did -- layer their existing GUI over a stable, secure base, like Unix.
Unfortunately, MS has "Not Invented Here" stapled over their half-brain, and will re-invent another square wheel instead.
Which is a shame, because they've got really good software engineers, and they could make a leap past Apple if they were willing to admit that Unix runs better than their creaking NT crap.