To apply the hack, create a text file with a .reg extension and the contents below:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SYSTEM\WPA\PosReady] "Installed"=dword:00000001
Run it by double-clicking in Windows Explorer. After this is done, if you run Windows Update, it will find several updates, as illustrated in this screen capture:
The screen capture is from our own tests, which appear to work. Notice that two of the displayed updates are for WEPOS (Windows Embedded Point of Service) and Windows POSReady 2009. The others are for Windows Server 2003, which runs the same kernel and many other components as Windows XP.
This specific hack works only on 32-bit systems, but a 64-bit workaround is available at this page, which may have been Williams's source for the 32-bit hack.
We asked Microsoft for a comment, but due to the holiday we don't expect one immediately. If we hear back from them we will update this story.
As we would anticipate them saying, Windows XP has reached end of service for a reason: its underlying technology is old and inferior to that of current Windows versions. Despite receiving security updates, it is not possible to secure the system properly. The systems being impersonated, WEPOS and POSReady 2009, are embedded applications that are not designed to be used like regular desktop systems.