Dual Booting

Hi I just bought and external hardrive for my Toshiba Satellite A-200
laptop and I was wondering if there was any way that I could dual boot
xp and vista with vista preinstalled. Because it is preinstalled I do
not have the boot disc. BUT. I do have the xp disc that I own. I was
wondering if anybody knew about a tutorial or any step by step guides on
how to install the xp os to the external hard drive and then dual boot
with vista. Thanks in advance to any help. I NEED HELP REAL FAST