Wish I could go back to those XP days. I always feel calmer whenever I see that wallpaper or the Mac OS X Leopard background. In reality I think it is just that I am tired of seeing flat design everywhere.
Yes you described it better than I could have. It is a kind of nostalgia for the past moments of peace. I was in High School during the XP era and going into college into the Leopard era. Both were times where I wish I could redo so many things but also times where things were so exciting about the future and life in general.
My laptop runs Xubuntu with Chicago95[0] and a high-res Bliss wallpaper[1]. (A complete mishmash, I know.) Too bad websites and other apps don't look that way.
That might be a pretty honest way to see which part of your life brought you the most peace.