Fortunately, you don't need to modify your Windows boot.ini to boot Windows from Grub. How about booting into Ubuntu, opening a terminal (applications > accessories > terminal), and do:
And for your Windows entry, replace it with:
gksudo gedit /boot/grub/menu.lst
If that doesn't work, let me know exactly what happens when you select it from the Grub menu on start up, and also post:
title Windows XP (hd1)
map (hd0) (hd1)
map (hd1) (hd0)
Otherwise let me know how it goes.