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.
[UPDATE:] Late Monday we received a statement from a Microsoft spokesperson:
We recently became aware of a hack that purportedly aims to provide security updates to Windows XP customers. The security updates that could be installed are intended for Windows Embedded and Windows Server 2003 customers and do not fully protect Windows XP customers. Windows XP customers also run a significant risk of functionality issues with their machines if they install these updates, as they are not tested against Windows XP. The best way for Windows XP customers to protect their systems is to upgrade to a more modern operating system, like Windows 7 or Windows 8.1.