Changing Desktop Environment by mistake

Hi,
I'm using fedora 15,
my defualt DE is XFCE I once saw that there is a option in startup DE's that I can select and it was OPENBOX. I just wanted to test it. but after choosing it as my DE here I am. I have just access to terminal, firefox.. what's in my desktop. there is no panel here. I want to go back to XFCE or Gnome 3.
when I restart my laptop it bypasses the startup login where I want to choose XFCE or Gnome.
please help me