The hack, as reported by ZDNet, fools Microsoft into thinking the system is running Windows Embedded POSReady 2009, a variant of XP that's used by ATMs and cash registers. Those systems will keep getting security updates until 2019.

Start “regedit” and add the following DWord to key HKEY_LOCAL_MACHINE\SYSTEM\WPA\PosReady: “Installed”=dword:00000001

On the downside, the hack only works on 32-bit systems. There's a workaround for 64-bit machines, but it involves manually downloading update files from Microsoft's website and tweaking them so that Microsoft doesn't block the installation. (Windows XP 64-bit is based on Windows Server 2003, which Microsoft is supporting until next year, and Microsoft checks to make sure XP users aren't installing those Windows Server updates.)

See Windows XP registry hack keeps security updates rolling for the dead operating system