Well slap my face and call me Sally.
Yesterday I posted an article about ditching XP because Microsoft was ending all support, including antivirus support, for that old and aged behemoth. I'll just go ahead and assume that they read my article, because that very same day they announced they will continue to support their security software for both enterprise and consumers through July 14, 2015.
So granny's machine won't turn into a botnet just yet (if it hasn't already). But that doesn't mean you're off the hook.