It sounds as though you have installed the xp system to the external drive.
In order for that to work your computer nust be able to support booting from external drives.

Go into the BIOS (press such and such key to enter setup)during initial start. Look through the BIOS optionns for boot devices and select to use external drives or something similar! Save and exit and see if you can then boot to the XP partion/drive!