Install GRUB loader when Debian fails to boot

mount /dev/sdXY /mnt

mount --bind /dev /mnt/dev &&
mount --bind /dev/pts /mnt/dev/pts &&
mount --bind /proc /mnt/proc &&
mount --bind /sys /mnt/sys

chroot /mnt

grub-install /dev/sdX
grub-install --recheck /dev/sdX

Disable service start up at re/boot - Debian 6

The following is the chkconfig equivalents on Debian 6:

update-rc.d  <service> defaults
update-rc.d  <service> start 20 3 4 5
update-rc.d -f <service>  remove

Part of the man page:

UPDATE-RC.D(8)                                   sysv-rc                                   UPDATE-RC.D(8)

NAME
       update-rc.d - install and remove System-V style init script links

SYNOPSIS
       update-rc.d [-n] [-f] name remove

       update-rc.d [-n] name defaults [NN | SS KK]

       update-rc.d  [-n]  name  start|stop  NN  runlevel  [runlevel]...   .  start|stop NN runlevel [run‐
              level]...  . ...

       update-rc.d [-n] name disable|enable [ S|2|3|4|5 ]

DESCRIPTION
       update-rc.d updates the System V style init script links /etc/rcrunlevel.d/NNname whose target  is
       the script /etc/init.d/name.  These links are run by init when it changes runlevels; they are gen‐
       erally used to start and stop system services such as daemons.  runlevel is one of  the  runlevels
       supported  by  init,  namely, 0123456789S, and NN is the two-digit sequence number that determines
       where in the sequence init will run the scripts.

       This manpage documents only the usage and behaviour of update-rc.d.  For a discussion of the  Sys‐
       tem V style init script arrangements please see init(8) and the Debian Policy Manual.