It's common for candidates to be experts in some technical area that the manager knows little about. Sometimes managers don't realize how to ask questions about the qualities, preferences, non-technical, or technical skills, so the manager asks, “Where do you want to be in five years?” Managers, if you want to know about ambition, ask another question such as, “When was the last time you wanted to improve a skill? What did you do?” instead.
Candidates, when you're faced with this question, consider your options. You can answer jokingly, “On a sailboat with a gin and tonic.” Of course, you run the risk that your potential boss will think you're a lazy lush:-).
A better answer is to help your interviewer by answering a question the interviewer didn't ask. Interviewers want to know about ambition, but they also might want to know how you've improved a particular skill, or worked differently with a group of people, or where you're trying to learn different skills. Maybe the interviewer wants to know if you need a technical track or a management track job. Take stock of your functional skills, domain expertise, tools expertise, and industry expertise, along with your non-technical skills and practice your answers before the interview.
Hopefully you've thought about your past jobs and how they've shaped your career. (If you haven't, read Jobs and Careers and Career Calculus.) Now, look at your progression throughout your career. Tell your story of the last five years: the experiences you had, the skills you learned, the lessons you learned, what you would do again and what you would not do again. When someone asks you where you want to be in five years, you can say, “Hmm, I'm not sure. Here's my progression through the last five years. (Now tell your stories.) I couldn't have foreseen this, but I'm looking to grow as a (developer, tester, writer, manager, whatever you are) and I'm willing to work hard to do so.”
Without telling the interviewer the question is a bad question, you've answered with a behavior-description answer — valuable information for the interviewer. You don't need a specific plan for the next five years, but you do need enough introspection to reflect on your past experiences to see where you've added value, so you can explain that to the interviewer.