1) From the messed up screen, hit CTRL + ALT + F1 which should bring you to a text only console.
2) login as your normal user
3) type sudo dpkg-reconfigure xserver-xorg (this will run an ncurses based step by step configuration of your graphical hardware)
4) when you get to the part about Video card, select "VESA", which is the generic linux video driver.
5) When it finishes, hit CTRL + ALT + F7, which should take you back to the graphical login (which should be fixed). If not, you can hit CTRL + ALT + Backspace to restart X, or type sudo /etc/init.d/gdm restart.