Blizzard says that support for Windows XP and Windows Vista will be phased out for a number of games. That means World of Warcraft, StarCraft II, Diablo III, Hearthstone, and Heroes of the Storm will not work with Windows XP and Vista anymore.
The news was posted in their forums:
Starting later 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.
After these older operating systems are no longer supported, the games will not run on them, 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.