the hard drive must be wiped before installing xp.
Boot off the xp install disc, and it should let lead you to a window with your hard drive partitions. delete all of them, and recreate. Reformat the drive, and finish the install.
I have heard that sometimes vista will block the xp install, you will need to boot off a linux live cd (knoppix/or Gparted are really good for doing things like this.) and then reformat using the disk tools in the os. then boot from the xp disc, and install.
Downgradng an os will require the drive to be erased, so make sure all data is backed up onto a backup drive, cd/dvds or uploaded to a server.
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