Win98 is easy, until you can't open the control panel without the whole thing BSODing.
It's fast, if you don't mind rebooting once every three hours.
And no, I don't mean that they're challenging, I mean that no matter what I tried, I could not for the life of me beat the last one. And THAT is frustrating--it should have been a side quest or something like that instead of these main missions...
And I probably could get it to work under XP, I'm just lazy and only tried it once.