You can only do this by a clean installation of XP Home. There is
no downgrade supported.
Why do you want to do this? Are you aware that Home is a subset
of Professional, and there is nothing in Home that's not also in
Professional. Unless you want to make the copy of Professional
available for use on another computer, there is no possible
advantage to doing this.
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.)