Formatting Windows Vista?

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I have reformatted many versions of windows, but I have not yet seen one like this. I have a Toshiba that came preinstalled with windows vista. I am wanting to take vista off and put on XP however when I try to format it says something about not able to format this volume. I have read on the internet that you need to create a partition or something. Can anyone give me step by step instructions to take vista off?