Counselor Question:

This is known as a negative question, one that interviewers love asking to test your ability to stay cool. We all have weaknesses. Let's face it, we're not all perfect. But that doesn't mean that yours should get in the way of your dream job. Think about how you can spin your weaknesses into something positive.
Look at the job description to hand and choose one required responsibility you feel you could improve on. Are you shy about making public presentations? Does your time management need a little work? Do you feel you would benefit from Microsoft Office training?
An example of the answer can be (depending on your weakness): "I am aware that my skills with Microsoft Excel and Powerpoint are not up to advanced level but this is something that I'm working on by teaching myself in my spare time."
Avoid being a perfectionist or simply saying, "I have no weaknesses I know of." This only makes you look arrogant and overconfident.