Again a simple question that you may not answer correctly. Most of the people just say that I need 2 weeks of notice period. It just shows that they are fed up with the current Job and it does not matter when they leave it. You have to answer it in such a way that it shows your importance.
If you are really in urgent need of a Job, then you can answer that I can join immediately.
Sample answer is:
I am working on an important project. I hope to finish it in 3 more weeks. I can give notice to my company and join after that.
I have travel plans between these two jobs. I will be travelling for 3 weeks after my current job. I can join your company after that.