Interpret the questions. What the interviewer really wants to know is what risks they are potentially taking by hiring you. It is also a stress question, and by asking you a stress question they can see how you react when it is thrown at you.
What They Don't Want To Hear
They don't want to hear that you have a huge weakness that can't be corrected under any circumstances, or something that would really negatively impact your performance on the job. They also don't want to hear about a personal trait that has no bearing on the job itself.
What They Do Want To Hear
They do want to hear you being honest about a minor weakness you have, and that you are doing something about it. You should turn the negative into a positive and show that you are not only aware of this weakness, but doing something about it.
Good Luck!
