to login from a terminal you can either press "Ctrl-Alt-F1" when the X login screen comes up or you can boot in to the "recovery mode" from the grub menu. If you pressed "Ctrl-Alt-F1" you can login as your normal user and use "sudo -i" to get a root shell, if you want with the grub option then you'll be root from the get go.
You should make sure you have space in /tmp and /home/$USER
that's normally the reason that that error message comes up.