One easy solution is not assigning a drive letter to it (via Administrative Tools/ Disk Management).
Another one is using a special bootloader, which can boot a hidden partition.
THAT one is free/opensource, has partition hiding abilities, works mighty well and is braindead to use, and absolutely safe, since it uses only the MBR and does not install any other files, anywhere.
Just curious though why you need two XP installations, and not just one and different user profiles.