Reinstalling only the GRUB in Linux

You may have Linux and Windows in your Computer and due to some reason you may have to reinstall the Windows OS. Now it rewrites your MBR and the menu has gone, now you can only boot Windows. Now in this case you need to install the GRUB again so that you can load both the Windows and Linux. Or for any other purpose if you need to reinstall GRUB then here is the steps:

 Boot into the live Ubuntu cd. This can be the live installer cd or the older live session Ubuntu cds.
When you get to the desktop open a terminal and enter.

STEP1:

sudo grub

This will get you a "grub>" prompt (i.e. the grub shell). At grub>. enter these commands

STEP 2:

find /boot/grub/stage1

This will return a location. If you have more than one, select the installation that you want to provide the grub files.

STEP 3:

Next, THIS IS IMPORTANT, whatever was returned for the find command use it in the next line

Code:

root (hd?,?)

Use the value from the find command i.e. if find returned (hd0,1) then you would enter

 root (hd0,1)

STEP 4:

Next enter the command to install grub to the mbr

Code:

setup (hd0)

Finally exit the grub shell

Code:

quit

That is it. Grub will be installed to the mbr.When you reboot, you will have the grub menu at startup.
Now the explanation.
Sudo grub gets you the grub shell. Find /boot/grub/stage1 has grub locate the file stage1. What this does is tell us where grub's files are. Only a small part of grub is located on the mbr, the rest of grub is in your boot folder. Grub needs those files to run the setup. So you find the files and then you tell grub where to locate the files it will need for setup. So root (hd?,?) tells grub it's files are on that partition. Finally setup (hd0) tells grub to setup on hd0. When you give grub the parameter hd0 with no following value for a partition, grub will use the mbr. hd0 is the grub label for the first drive's mbr.
Quit will exit you from the grub shell.