First of all, you should log in into single user mode and remount required partitions with
Booting into single user mode
Interrupt boot process to access GRUB menu. In order to do that, press any key when 'Booting into .... in X' is shown.
Enter edit mode of your latest kernel (or the one that you're using as default) and edit line which starts with
linux /boot/vm* <..> ro quiet splash and append
init=/bin/bash to the end of it.
F10 to save and press
b to boot with the new option.
This will boot you into root shell (you'll have no GUI there). Now remount root as writable filesystem.
mount -o rw,remount /
Eliminate the issue
+- Investigate the issue directly from single user mode.
Find the broken script in
/home/guest/.bashrc or any other configuration file which is sourced at the log in process.
+- Disable guest from auto logging in into the system by default.
Resuming boot process
Remount root filesystem into
ro mode with
mount -o ro,remount /.
/sbin/init to continue the boot process, or press
CTRL+d to reboot the machine.
Depending on which path you took, you'll be able to log in either as guest (if stalling script was eliminated from auto execution on log in) or as your user with root privileges (if guest was excluded from autologin process).