If u have a dual booting machine:linux and windows on the same machine
and u have installed windows again then the MBR(master boot record) of
the machine which earlier contained GRUB will be overwritten by the
windows MBR so to get u r GRUB back one has to follow these instructions:
1.Boot the machine from the linux cd
2.on the prompt type:"linux rescue" and then press enter
3.After telling all the keyboard and language specifications it will
lead to shell prompt
4.Once the prompt is there then on the prompt
type:"chroot /mnt/sysimage"
5.then type:"grub-install /dev/hda"
6.After this command the grub will start installing and once it is
done then type:"exit"
and the machine will reboot
After the above process u will see u r GRUB back on its place.