Download
Knoppix and burn it to a disc, boot to it.
Mount your Ubuntu drive and chroot into it...
Code:
mount /dev/whatever /mnt/ubuntu
mount -t proc none /mnt/ubuntu/proc
mount -o bind /dev /mnt/ubuntu/dev
chroot /mnt/ubuntu
This will put you into a command line as if you were booted into Ubuntu. First you need to understand how grub labels disks and partitions... (hd0,0) is the first partition on the first drive, (hd0,1) is the second partition on the first drive and (hd1,3) would be the 4th partition on the 2nd drive. Basically it's (hd<drive>,<partition>).
Actually, I need to know your partition layout before I can help give you exact instructions, so boot to Knoppix and run
Code:
cat /proc/partitions
and paste the output here. Once you do that I'll be able to give you step by step instructions.