It's so weird, I can't help but feel nostalgic when I see Windows XP screenshots. It seems I really like it's aesthetic. I'm quite sure it wasn't the case at the time.
Windows 95 does it for me. I still remember how advanced it looked like compared to 3.1! The XP default theme on the other hand still feels like a fairly unsuccessful and in some way infantilizing attempt at eyecandy. Around here at least it was commonly known as the ”Teletubbies” theme.
I always used it in "windows 2000" skin mode to save on computer resources. Can still vividly remember the day I got a new computer with enough RAM that running the Win Xp default theme didn't really make an impact.