In the last link of my signature
Task B1 shows how one can restore XP by a bootable DOS floppy
Task B2 shows the same but using XP installation CD, same as Grips suggested.
To edit Grub or Lilo is also dead easy. On any non-bootable Linux just use a Linux Live CD, boot it up and opt for superuser (by typing "su") to get root privileage.
Grub's configuration is always stored in /boot/grub/menu.lst
Lilo's equivalent is stored in /etc/lilo.conf
One just mount the Linux partition and edit one of these two files, save the file and then reboot. For Lilo there is a complication that one must issue the "lilo" to recomplie the boot loader and that the user normally needs to change-root into the affecting partition.
I use vi for editing as it is available in every Linux.
In the case of the original poster xubuntu is avilable and presumably bootable then it is just a matter of editing its /boot/grub.menu.lst and make sure it has these lines for booting Windows, which I assume to be the first partition of the first hard drive or (hd0,0) in Grub's term
Code:
title My Windows in 1st partition of 1st disk
root (hd0,0)
chainloader +1
Just don't forget root privileage is needed to amend a Linux system file which is far better edited in the terminal mode.
Grub counts from 0 so the 1st disk is (hd0) and its 1st partition is (hd0,0)