While trying to login into primary account, I used to get black screen with movable cursor. Now, every time I try to login to that account, it keeps sending me back to login screen. But, when I login using guest account, every thing works fine. I couldn't even solve the problem reinstalling ubuntu-desktop and lightdm. I think unstalling whole graphical interface and reinstalling it will fix the problem. Could someone tell me how to completely uninstall ubuntu graphical interface and reinstall it?