The GNU GRUB is the program that boots Linux, and its configuration file seems to be corrupted.
In order to see what's going on, I will have to see the contents of this file, which resides at /boot/grub/menu.lst.
Go to backtrack-linux.org. Click downloads, read the bottom of the page for instructions. (You can skip registration by clicking "download" within filling out the form on the download page).
Once you have burnt it to a cd, put it in the cd drive of the broken computer, boot the cd. Your username will be "root" and your password is "toor".
fdisk -l /dev/hda
fdisk -l /dev/sda
(hit enter after each line)
This will describe the way the hard disk is laid out.
Post the output here. We can then (hopefully) help you locate the configuration file and repair it.
to shut the conputer down.