Question Details:

Windows notified me that I did not have a genuine copy of "Windows XP Professional" on my computer. So I purchased a "Windows XP Home Addition" on line. When I went to install, I got that error message.

Answer:

You received that error because Microsoft doesn't support that kind of upgrading from Windows Pro to Windows Home. Windows Pro is considered a superior version than Home Edition because it contains more features.

In order to install Windows XP Home Edition you must do a clean install. The clean install will erase all your information on your system partition. Backupt your date first.

Details on how to do a clean install are here.