If you are struggling with dual-boot problems and Grub 2 this may help.

1) Boot from cd or flash drive with the same architechture ie. 32 or 64-bit as your missing o/s

2) Get to a root console prompt

3) type:  blkid 

this gives you a listing of all your partitions with newer sata drive hardware

                                                            or        type: fdisk -l

for older ata style machines.

4)locate the operating system partition eg. sda1 sdb2 sdc3 etc.

5) mount this partition type: mount /dev/sd** /mnt

(** whatever partition your o/s is on - discovered from the blkid command)

6) Create a chroot environment,

type: mount --bind /dev /mnt/dev

mount --bind /dev/pts /mnt/dev/pts

mount --bind /sys /mnt/sys

mount --bind /proc /mnt/proc

7) type: chroot /mnt

you are now using your previously unbootable operating system. 

Edit: sometimes this may exit with an error, if this happens, change the command to : chroot/mnt /bin/bash

8) type: grub-install /dev/sd*

(* whatever drive - not the partition your o/s is on - discovered from the blkid command eg. /dev/sda)

9)type: update-grub

this should list all the o/s's grub can find and install to the mbr (master boot record) of the boot drive.

10 type:  cntl+d (control key and letter d) this exits the chroot environment and brings you back to the cd/flash drive system

11) exit gracefully by unmounting type: umount /mnt/dev/pts

umount /mnt/dev

umount /mnt/sys

umount /mnt/proc

umount /mnt

12) reboot

you should now have a restored and working grub boot loader.

Good luck to all, Tony