If your ubuntu is (hd0,3) then this should do it from a liveCD:
In terminal
sudo grub
root (hd0,3) <-- this must point to location of 'stage1', else see nb:
setup (hd0) <-- installs grub IPL to MBR of first hard drive.
quit
Reboot
nb: if location of 'stage1' is unknown,
find grub/stage1
or
find /boot/grub/stage1
is needed, after step #1
http://ubuntuforums.org/showthread.php?t=1285499
