Thank you, Larry Seltzer of Zero Day
First things first. Microsoft’s response to his hack:
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.
Now, to those still interested. You can hack the registry in Windows XP and trick Windows Update into continuing to send you security updates. The basis of it is that Microsoft has an embedded variant of Windows XP and support doesn’t end on that until April 2016. The hack makes XP look like the embedded version.