This question gets asked in pretty much any job interview. Research the company's mission statement and values. Make sure you don't list one of its areas of emphasis as one of your weaknesses. Do be truthful and emphasize that you're working on your weakness. Portray it as positively as you can.

Example: "I get so engrossed in work projects that I tend to focus on work too much-coming in early and staying late. It's definitely something I'm addressing. I recognize that the best employees have a healthy balance in their lives. I have a new strategy that includes never taking work home with me on the weekends."