IceWM with WinClassic, a Windows 2000-lookalike theme, which shouldn't be hard to knock around until you got something Win98 or even Win95-ish. Or WinME-ish, if you're a glutton for punishment.
Now Windows 3.1, that's sheer madness. Utterly ridiculous. Total insanity.
Post screenshots when you've got them. If you build it, they will come.