Boot the Ubuntu live CD and get to a command prompt. Type: sudo fdisk -l
to see what is your Ubuntu boot/root partition. If it is /dev/sda2 (the second partition on the first drive), type the four commands below to open a grub session and reinstall grub in the MBR of the first hard drive set to boot in the bios:
Shutdown the live CD session, remove it from the optical drive and reboot the computer, you should be able to get a menu with both XP and Ubuntu.