1>Method One - Boot from Installation CD
boot from the linux CD.
type this at the boot prompt:
linux rescue
use the ensuing rescue procedure to get to a command line, with your / mounted, say as /mnt/rescue
then say this:
chroot /mnt/rescue
now your / on your HDD is mounted as / and not /mnt/rescue (formerly, / was the root of the RAM drive your CD boot sets up.)
Now, type this:
/sbin/grub-install /dev/hda
now this
exit
exit
and reboot.
2> Method 2 - Boot from rescue floppy
once the floppy is booted, and you have logged in:
/sbin/grub-install /dev/hda
now reboot.