Taken from: http://www.motorhomefun.co.uk/forum/threads/how-to-get-windows-xp-updates-until-2019.114257/
Copy the lines below to a text file.  Save the file with a .reg extension.  Then double-click on the file you saved and choose to run the file.

Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\WPA\PosReady]
"Installed"=dword:00000001