An interim report which examines adoption and deployment issues associated with Microsoft's Vista and Office 2007 products. [via Becta - About Becta - Publications - Microsoft Vista and Office 2007: Interim report with recommendations on adoption and deployment]
The report states:
"Windows XP was developed from an existing operating system whereas Vista is a wholly new operating system."
Now I would have said that was twaddle - Vista has an entirely new kernel? Where do you draw the line between operating systems and applications running on top of that - it looks to me that there is a lot of legacy stuff installed with Vista.
Maybe I am wrong and it is completely new - in which case it is astonishingly stable.