I'll assume that the ntfs partition will not be the first partition on the drive, i.e., you will make the partition at the end, behind Linux.
After making the partition, and coping xp to it, edit /boot/grub/menu.lst (grub.conf on some distros).
Add something like this:
map (hd0,0) (hd0,1)
map (hd0,1) (hd0,0)
Here, I assume the partitions are hda1 and hda2, with xp on hda2. Windows will not boot unless it's on the first partition of the first drive. So, we tell it a little lie, that it's number one when it isn't. That what the map lines do, tell windows it on the first partition of the first drive.
That should be all that's required to add xp to the grub menu, and make it bootable (assuming the partition is flagged as bootable when you make it).