Boot from Ubuntu 9.10 Live DVD or CD.
Assuming the Ubuntu partition is sda3, and /boot partition is sda4 (if you have a separate /boot partition), open terminal and run:
Code:
sudo -i
mount /dev/sda3 /mnt
mount /dev/sda4 /mnt/boot #skip this if don't have a separate /boot partition
grub-install --root-directory=/mnt/ /dev/sda
You can find information about partitions with GParted.