I now just realized that my XP Pro install CD is the upgrade version and not the full version. It never ends does it?
Did you use it previously for an upgrade? If so, you should have an old Windows 98, Me, NT 4, or 2000 CD. When you install XP Pro, it will want to see one of those CDs during the process.
Boot to your XP CD. It will ask you to insert your qualifying previous Windows CD (98, Me, etc.) to verify that you upgraded. During the process, it will ask you which partition to install on. It will see that there is already a version of Windows on the disk, and that's fine. You'll install to that partition anyway, acknowledging the warnings. Do not delete the partition, just install to it and overwrite the version of Windows (Vista) that it sees is already there.
You do not need to reformat your hard drive! Your data can sit there happily as you downgrade back to XP. You'll just have to reinstall most applications (back to the same folder where they were installed before) to make them work again after XP is reinstalled.
Go through the install process and it will go back to XP. You'll need to reinstall hardware drivers (video, sound, network, modem, etc.) and applications, but your data files should be right where you left them on the hard drive.
Hope that helps. :-)