If you can boot off a live CD and get to a command prompt you can do this - assuming you will use grub to choose between XP and Linux
Code:
cd /tmp
mkdir ubuntu
mount /dev/<WHERE UBUNTUU IS INSTALLED - like hda6 fex>
chroot ubuntu /bin/bash
source /etc/profile
mount /boot
grub
At this point enter these grub commands - note hd0,0 refers to first disk, first parition - change accordingly to where grub is installed
Code:
root (hd0,0)
setup (hd0)
quit
Now reboot like so
Code:
umount /boot
exit
umount ubuntu
reboot