IIRC Vista had a HIG document to par with Apple's and it's not surprising because they really wanted to change interface and kept it unified. Which worked for the most part but event today, in flat style era there are still 9x widgets deep down below that cannot be rooted out. What they could do was flattening these down.

But with release of Office 2007 and Windows Live suite they started changing interface again and Windows 7 was based upon slight plastic flatness and ribbons all over the place.

There was a community ran project Windows Taskforce that tried to catch up all sorts of interface quirks and with provided mockups they wanted MS to further polish their flagship project between Vista and 7. Sadly all these efforts were gone when MS decided to go Metro in Windows 8.