We don't really read them, do we? We just want to get them out of the way. I think Cancel/Shut Down is always better than No/Yes, which forces me to read what the question is about. Of course, in an extreme case like "Cancel this reservation?", it's getting complicated... that might be the exception, fair enough.
I would even shorten the title to a more economic "Shut down your computer?". The first part "Are you sure you want to" is noise. It's not friendly sugar. As to "Now", it's pretty implicit.
