This is a very common problem. You probably set wrong parameters for your video card or monitor. If you receive a log-in prompt after the error message, simply log-in as root, and type "xf86config". Make sure you have all the specs for your monitor and graphics card available. Always test the settings if asked. If it wont test properly then go back and redo some settings. You can also log-in to a console at boot by typing "linux init 3" (without the quotes of course). If asked to startx at reboot say yes. When you redo the settings and reboot, if you do not get a graphical login, just type startx and that will bring you to kde. I would set resolution at 1024 / 768 and color to 256 for sure to get going. Go here for more info.
http://www.mandrakeuser.org/docs/xwin/index.html