Since there is no way to downgrade a Windows XP Professional installation back to Windows XP Home Edition, a "clean install" would be required which requires a reformat of the hard drive. The easier solution is to consider purchasing a new Windows XP Professional Product Key so that you can activate and still retain your current XP Pro installation, programs and files.
Microsoft offers an additional license service to customers who have legally installed Windows XP and have an immediate need for an additional license due to work stoppage or other unforeseen circumstance. In the U.S. and Canada, call the Windows Product Activation Call Center at 877-426-6010 for more information on how you can take advantage of this service.
Clean Install Windows XP:
http://www.michaelstevenstech.com/cleanxpinstall.html
Using the Files and Settings Transfer Wizard in Windows XP
http://www.microsoft.com/windowsxp/using/setup/expert/crawford_november12.mspx
HOW TO: Use Files and Settings Transfer Wizard in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;293118&Product=winxp
List of Programs Whose Settings Are Migrated When
You Use the Files and Settings Transfer Wizard
http://support.microsoft.com/kb/304903/en-us