It came with Vista Premium Home. I installed XP Pro
How did you install XP? Into the same partition as Vista? You didn't remove the vista installation?
I thought with Vista installed it won't allow XP to install - you have to remove the Vista partition first.
Although Vista's boot mechanism is different to previous versions of windows, the usual rule applies for dual booting using MS's boot manager - older o/s has to be installed first.
Not knowing what you've done or level of expertise, my dual boot advice is:-
1. Use a third party boot manager - I use
others like Xosl, boot-it.
2. Installations on separate hard drives if possible, otherwise primary partitions (not extended with logical drives).
3. Installations should be independent. If using different drives, install second o/s with first drive disconnected. If using separate partitions, need to hide the first partition when installing second (various apps can hide partitions - including boot-us).
If your machine had a single partition with Vista on it and only one hard drive, you really needed to either obtain second drive or shrink the vista partition to make room for XP (2nd drive far superior option).
If you can tell us how you installed XP, we may be able to help - though if Vista is gone, it will obviously need reinstalling/restoring from recovery disks/partition if you want dual boot.
PS. Have you considered using virtual machines - MS Virtual PC is free now.