Microsoft has released preliminary (version 0.7) UX guidelines for
upcoming Windows Vista. You can get them at http://tinyurl.com/9566c
It's (obviously) unfinished and rather messy, but contains some
(blurry) illustrations and gives the general taste of things to come.
Be warned that what you'll get is a .exe file that uncompresses to a
local set of HTML pages. Also, you can't even download it from
anything else than a genuine Windows system. I've copy-pasted the
contents for those who might have problems getting it:
I.