Perhaps you are using the gdm login manager and need to configure it?
Gdm - GNOME version of xdm
I prefer using a console login, using startx
to start the GUI. I create a file .xinitrc in my home directory to do that stuff.
If you would like to do that, edit your /etc/inittab and change the default runlevel to "Full multi user mode" and create a file similar to mine:
# Duron box "uilleann" user fancy .xinitrc file
# Start the stuff I want to run when I startx
# The artwiz-fonts sometimes conflict with your locale settings. To get
# them to work you may have to put the following at the beginning of your
# .xinitrc file
# export LC=C
# export LC_ALL=C
# Set the background for fluxbox
# Esetroot ~/backgrounds/linux.png &
# Start gkrellm and put it in the slit
gkrellm -w &
# This will run a screensaver as your background
# /usr/X11R6/lib/xscreensaver/hopalong -root &
# Start the screensaver daemon
# Finally, I start my window manager
# exec kde
# exec gnome-session
If you prefer to stick to your gui login, try a search of the board on setting up gdm (or xdm or kdm, whatever you use) to switch to another DE/WM.