It's taken me a long time to figure these out. They're roughly in order, although it depends.

• "What are you trying to do?"
  • It's harder to help people when I don't know what they're trying to do - and it's even harder when they don't know either.
• "Why do you want to do that like that?"
  • Often there's an easier or more direct way.
• "What is the computer (not) doing?"
  • This question determines what the problem is, but it's better asked after I know the person's intentions.
• "How are you trying to do [what you want to do]?"
  • Sometimes people can't explain how they're trying to do it, but it's important to know the answer to this question.

And, if one is not speaking to someone with computer experience, the question one does not ask is: "Have you looked at the help documentation?" Because none of them have.