Re: Just what were you running exactly???
You'd be crazy to run one of the more demanding DEs on a machine from the XP era anyway.
I found Gnome-shell the hardest resource-hit of Unity, KDE and Gnome. Mostly usable on a 2010 single-core AMD custom with 4GB Ram, except with video playback, which saw system struggle, may be due to underpowered embedded graphics, but KDE and Unity did not have the same issues.
14.04 / 14.10 Unity runs great on an Atom N450 netbook with 2GB Ram, not quite so well with 1GB less though (dash,hud,workspace activation delays hampered its usability).
If you don't have the whack for these three (or the several newish OSX-ish Gtk3 shells), theres, LXDE, XFCE, or Openbox, Blackbox, Fluxbox, WindowMaker, Pekwm, FVWM, JWM, Awesome, XMonad, i3, Herbstluftwm, Subtle, dwm, Scrotwm, Sawfish.......(or CLFSWM if you are really adventuresome).