XP should be on Grub when u install it, unless u installed it on a diff HDD like Zorch said. Just add it onto your grub menu. Thats what I did. And if you use XP more make it the default option. I can't remember how to actually add it but you add it here: \boot\grub\menu.lst
You have to SUDO to that location with gedit or it will not save, must be opened with root privledges.