Ok if you have done what P1ngPong said to do, insert the cd, then after POST (this is before the computer starts the XP boot sequence) the computer should automatically ask if you would like to boot from the cd (or "press any key to boot from cd"), and after that it should be pretty straight forward.
I really hope you get your problem solved.
EDIT: is an external CD/DVD Drive, or a external HDD?
I really hope you get your problem solved.
EDIT: is an external CD/DVD Drive, or a external HDD?