I'm feeling a little dense today, so bare with me
"IF" I'm understanding you correctly, You have a laptop that currently has Vista installed. You also have an old HDD that has XP, all your files and apps installed and you wish to plug in the old XP HDD externally to the laptop and simply boot XP up from the external drive?
Which PC did the added external XP drive come from? Was it the same laptop or a completely different PC? If it was completely different PC, then it won't be possible to boot into XP on the laptop as all the configurations and settings will not be correct.
Your best bet would be to partition the non-external HDD in the laptop and dual-boot Vista and XP. Unfortunately this will require a fresh installation of XP and your apps.
Can you currently access your valuable files on the external HDD in Vista? If not, is it possible to put the old XP HDD in the old PC and copy your files into another folder.
If the XP HDD came from the same laptop, swap the Vista drive with XP drive and copy your valuable files to a another folder that can be accessed by Vista (as outlined by norm and the Shadow). Then go ahead with dual-booting.
If I've completely missed the point, I do apologise
