From the mainstream stuff, XFCE might be close after some configuration (especially if you install some of the more Windows-like themes) but MATE/GNOME2 isn't really that XP-like. Trinity (KDE 3.x fork) would be closer though.
As mentioned in another reply IceWM would also be very close - at least compared to classic Windows - but that only gives you the taskbar and window themes.
As mentioned in another reply IceWM would also be very close - at least compared to classic Windows - but that only gives you the taskbar and window themes.