Check on your display manager screen (the login screen) for any
identifying marks: does it say GNOME Display Manager? or K Display
Manager? In either one you should have a menu marked "Sessions" ---
in KDM it's hidden under the "Menu" button next to the "Login"
button. You should find a Menu that says "Default, Custom, KDE, GNOME, Xfce, Fluxbox,
WindowMaker, Enlightenment (if you're lucky
" Use this menu to
choose which session (desktop) you want. They are all there.