How to restore Grub from a live Ubuntu cd.
This will restore grub if you already had grub installed but lost it to a windows install or some other occurence that erased/changed your MBR so that grub no longer appears at start up or it returns an error.
(This how to is written for Ubuntu but should work on other systems. The only thing to take note of, when you see "sudo" that will mean to you that the following command should be entered at a root terminal.)
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. (I am going to give you the commands and then I will explain them later)
Next, THIS IS IMPORTANT, whatever was returned for the find command use it in the next line (you are still at grub>. when you enter the next 3 commands)
Next enter the command to install grub to the mbr
When you reboot, you will have the grub menu at startup.