The only way to change from WinXP Pro to WinXP Home is to format
the drive and start over. There is no supported downgrade path or
technique.
Simply boot from the WinXP Home installation CD. You'll be
offered the opportunity to delete, create, and format partitions as
part of the installation process. (You may need to re-arrange the
order of boot devices in the PC's BIOS to boot from the CD.)