For all devices running Windows or anything else...
If we're going to count them, I need to see how to get to a command prompt. If I can't, it doesn't matter if it's Windows, Linux, or the fictional OS on hacker and government computers in TV and movies.
If that definition doesn't work for you, here's a better one - if I can't *change* which OS the device runs, never mind choose from any, we shouldn't be counting it. Java may be in your ATMs, gas pumps, toaster, and battery operated vibrators, but it doesn't matter if someone outside of the device has to tell you this, and the same goes for the OS.