Pop the Ubuntu CD into the drive to boot it up.
Click terminal, then type
take note of the first partition with Type 83, my guess is partition 1 if partition 0 has Windows inside. Assuming the "/" of your Ubuntu is in this partition known to Grub as (hd0,1) then Grub will be back if you type
The grub-install /dev/hda
suggested by Vincent Vega also works but you need to mount the UBuntu partition first and "change root" into it. It is described as Task B4 in the last link of my signature. The retoration of Grub by a Grub shell above is labelled as Task B4 in the same link.