I sometimes come across questions on SO about someone having trouble installing a piece of software (recent example here). Since installing software sounds to me like a general computing topic, not a programming topic, I usually suggest that it belongs on SU.
I've seen comments from those who don't agree saying that it's appropriate to ask such a question on SO if the software in question is directly related to programming (like an IDE). When I looked up this poll question about appropriate programming-related topics, the answer that seems closest to the issue of software installation is this one, which is comparatively pretty low on the list (#12 by votes) of what people feel is approved SO content.
I thought I would ask this question to more specifically address the issue:
Under what circumstances is it appropriate to ask questions on SO about software installation problems?