I have some nostalgia for XP, especially the Zune theme (separate download, black+orange recolor of the default), but due to the Classic theme being available in so many versions and often using it either for more performance or easier ricing (can easily swap the colors and fonts via official settings), I'm also nostalgic for the Win95 or so UI. I think 2000 was the oldest I remember actually using, but I used XP a lot and 2000 not very much.

In the last decade+ of using GNU/Linux, I've also become very attached to bitmap fonts and simple solid colors, while I've grown to dislike curves and transparency. So sometimes I see a screenshot of some very old Mac OS version I never even used, and it just looks good, sharp, and clean to me, no real nostalgia involved.

I think SerenityOS's vision of a unix-like environment with classic Windows UI is genius. I don't follow the project that closely, but on paper it does seem like a good idea.