Windows XP has effectively been put to pasture by Microsoft — almost. If, for some reason, you're still hanging onto this relic of an operating system, it is possible to get official security updates from Redmond itself until early 2019 with a simple registry tweak.
The updates aren't for your regular brand of Windows XP for desktop, they are in fact issued for "POSReady 2009", an offshoot of the OS designed for point-of-sale hardware. As Martin Brinkmann explains on gHacks, this isn't a magic bullet for keeping XP watertight against internet baddies — there's a possibility the updates could mess up your installation and there's nothing stopping Microsoft from devising a way to prevent the updates flowing to systems it wasn't designed for.
Also, the updates are aimed at the 32-bit version of XP, but it is possible to get them working with the 64-bit flavour, with some tomfoolery (link is in German).
With those provisos out of the way, here's what you need to do to mark XP as the POSReady version. Crack open a new file in Notepad, name it whatever you like as long as it has the extension ".reg" and paste the following into it:
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SYSTEM\WPA\PosReady] "Installed"=dword:00000001
Save the file, double-click on it and accept the changes to the registry. That's all there is to it, other than waiting for the updates to roll in.
Again, it's hard to recommend this over a fresh install of Windows 7 (or 8.1), but if you're desperate, it's better than nothing.