For example after install Ubuntu 9.10 and then I install Windows operating system along with Ubuntu. The windows will override Master Boot Record on Hard driver , so from now when your computer boot up, you only can see Windows.
To solve the problem:
We need to have a CD version of Ubuntu 9.10. Boot our PC with live mode of Ubuntu 9.10
.Open Terminal ( Applications > Accessories > Terminal)
(We suppose that the Ubuntu already installed on partition sda3 of sda)
sudo -i (change user to root user).
mount /dev/sda3 /mnt
grub-install --root-directory=/mnt/ /dev/sda
mount --bind /proc /mnt/proc
mount --bind /dev /mnt/dev
mount --bind /sys /mnt/sys
chroot /mnt update-grub
Reboot your computer and enjoy !