Answers: is, because Windows is a proper noun to be precise singular, but window is not a proper noun, plus it is a plural
if you look at it in proper english the word (windows) seem plural and the proper sentence should be
Windows are shuting down.
But in computer spoken language its considered as individual one single piece. window is a operating system. so its one not more than one.
So the proper process to vote is
WINDOWS IS SHUTTING DOWN.
english is not my first dialogue but i regard I made it clear.