http://support.microsoft.com/default.aspx?scid=kb;en-us;329261
"Service Pack 1 Setup Canceled" Error Message When You Install Windows XP
SP1
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q326904
Error Message: The Product Key Used to Install Windows Is Invalid
Checking the Product ID
During the installation of Windows XP Professional, you are prompted to
enter a 25 digit Windows XP Product Key, which Windows XP promptly converts
to a system product ID. Because of security concerns about piracy, Microsoft
does not provide a tool that allows you to view the Product Key (or CD Key)
that was used to install the operating system. However, Windows XP Service
Pack 1 ships with a list of the two product IDs that are created by the
pirated product volume license product keys.
(The Product ID can be found by right clicking My Computer and choosing
Properties) To determine eligibility for the update, Service Pack 1 compares
the Windows XP product ID on the system to this list.
The comparison and the list reside locally on the users PC and no
information is sent to Microsoft as part of this process. Service Pack 1 for
Windows XP will fail to install on installations of Windows with one of the
following product IDs: XXXXX-640-0000356-23XXX and XXXXX-640-2001765-23XXX
http://support.microsoft.com/?kbid=328874
HOW TO: Change the Volume Licensing Product Key on a Windows XP SP1-Based
Computer
http://support.microsoft.com/default.aspx?scid=kb;en-us;303661
How to Uninstall Windows XP and Revert to a Previous Operating System
http://support.microsoft.com/default.aspx?scid=kb;EN-US;314052
How to Manually Remove Windows XP and Restore Windows 95, Windows 98, or
Windows Millennium
Purchase Windows XP Pro (Win XP Home won't work) and perform a Repair
Installation using the new valid key and CD:
http://www.dougknox.com/xp/tips/xp_repair_install.htm