You should have a ~/.xinitrc file. Open this in your favorite text editor (after creating a backup of course!).
It should have a line which says "exec startkde" or something. Change this to any of the following to change your default login.
KDE = startkde
Gnome = gnome-session
Blackbox = blackbox
FVWM = fvwm
FVWM2 = fvwm2
Window Maker = wmaker
IceWM = icewm
Fluxbox = fluxbox
So for example, if you wanted GNOME as your default, change "exec startkde" to "exec gnome-session."
In the same vein, from your text-based login, you should be able to execute any of the window managers as follows:
I'm not sure about this, but I think this is the way that I did it back when I had a text-based login. Also, if the command doesn't work, maybe your version of Linux requires a direct path to the executable.
On Debian, installing Fluxbox would require:
and uninstallation would be:
Tell me if this works.