Boot off a live CD, then in a terminal window enter, (substituting your (drive,partition) pair):

sudo grub
find /boot/grub/stage1
root (hd0,0)
setup (hd0)
quit

Reboot the system

As root edit /boot/grub/menu.lst, it's fairly self explanatory.

Boot off a live CD (gparted is quick), then in a terminal window enter, (substituting your (drive,partition) pair):

sudo grub
find /boot/grub/stage1
root (hd0,0)
setup (hd0)
quit

Then reboot the system

As root edit /boot/grub/menu.lst, it's fairly self explanatory.