I get this question a bit, and it was asked again today on the Stanski list: "Where can I find UI guidelines for Windows Development?"
For Vista: http://msdn.microsoft.com/windowsvista/uxguide (see also http://msdn.microsoft.com/windowsvista/prodinfo/top10/default.aspx)
For XP: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwue/html/welcome.asp
Note that the XP link has a big graphic that says something like "Buy This Book!". You don’t have to buy the book, it’s all online. But if you want hard copy, then that’s available too.
You should also look at the SSW site:
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