Reverting Back to XP Home Without CD or Recovery Partition?

I need to revert my old Toshiba's Satellite laptop with Windows XP Pro back to the Windows XP Home edition it came with. The Pro edition does not pass validation (I was given this computer by my grandmother; I suspect the person who had it before her, another relative, did not know of Windows rules of multiple installations.)

I know that this system originally came with XP home, because there is a sticker on the bottom of the laptop with a Home edition Serial number on it -- unfortunately for me, I have neither the recovery CD, nor does there appear to be a recovery partition (no i386 file is present on the system.) I'm not sure if this was an upgrade or clean install of Pro edition.

Essentially, I have a non-genuine copy of Pro installed, and the original Home product key -- can anybody help me?