Problem: Install ubuntu first then install Window XP. As a result, the GRUB is overwrite by Window Bootloader.

Solution: Reinstall GRUB.

Boot into ubuntu using the ubuntu disk.

After that, open the teminal.

Type the following commands.
sudo grub
find /boot/grub/stage1
root (hdX,X) – replace X with the value that the find command return. Example (hd0,0)
setup (hd0)
exit