Blizzard have apparently announced that, from October this year, they're going to start withdrawing support for their games on Windows XP and Vista. Once that support has gone, they state those games will simply stop working. So if you're still running on one of those systems, it's time to upgrade
Starting in October of this year, we will begin the process of ending support for Windows XP and Windows Vista in World of Warcraft, StarCraft II, Diablo III, Hearthstone, and Heroes of the Storm.
Microsoft ceased mainstream support for these versions of Windows in 2009 and 2012, respectively, but since a decent portion of our audience was still using them at the time, we continued supporting them. However, there have been three major Windows releases since Vista, and at this point, the vast majority of our audience has upgraded to one of the newer versions.
The games will not run on these older operating systems once they are no longer supported, so we encourage any players who are still using one of the older OSes to upgrade to a newer version. We’ll be rolling out this change on a staggered schedule, and will post further notices as we get closer to making the change for each game.