Hi,
Boot the system with your XP Home media. At the point of setup where you
choose the installation point, delete the existing system partition, then
create a new one from the free space. Format, proceed with installing XP.
There is no way to revert an existing Vista upgrade back to WinXP or to
install WinXP over Vista, you must create a clean installation. The only
exception would be if you had previously imaged the XP installation, but
then you would have known if you did that and would use that same software
to recover the original installation.
If your WinXP software came preinstalled on the system, you need to consult
with the manufacturer with regards to the proper way of using the supplied
recovery media as the instructions vary, particularly if the recovery
information is on a separate volume of the hard drive.
--
Best of Luck,
Rick Rogers, aka "Nutcase" - Microsoft MVP
Windows help -
www.rickrogers.org