How will you be installing Windows XP? Do you have a slipstreamed CD or some copied files on a partition? If you actually have a bootable WinXP SP3 CD, then you don't have to worry much. Just boot with it...and when it asks you to setup the partitions, format the one having your Vista installation.
Follow the following link for a detailed guide on how to do a clean install of Windows XP
XP Clean Install