LinuxQuestions.org - External ubuntu install on external USB HDD

I've been trying to get a dual boot going on my desktop but do not want to wipe out my windows XP install at the moment. I didn't have much hard drive space so I decided to install the latest version of ubuntu on an external usb hdd and was hoping to dual boot of of there. However when I boot up my computer with the usb drive on it still dosnt boot into ubuntu. I've tried changing the boot priority however when I change it to boot from the external drive I just get a message asking to insert the media for the OS to boot. At this point if I insert my ubuntu disk, then when it loads up I click on boot from hard drive, it will return the same error. How can I make it so I can boot up into the ubuntu OS and still be able to access my XP when needed?