Personally, XP is much better than 7.
It's not that user friendly but most people gotten used to it already and simply easier to use compare to windows 7...
I'm quite stumped when people never mentioned to install Vista when they have either XP or 7 on their system...
Windows 98/ME is very old and I'm sure you already know and I also have used win98 so I can't also forget it. Easier to tweak, simple environment and most recent software still support 98. But XP has already steal it's glory so it's best to put it to bed...