MDI (Multiple Document Interface) is a Microsoft Windows programming interface for creating an application that enables users to work with multiple documents at the same time. Each document is in a separate space with its own controls for scrolling. The user can see and work with different documents such as a spreadsheet, a text document, or a drawing space by simply moving the cursor from one space to another.

An MDI application is something like the Windows desktop interface since both include multiple viewing spaces. However, the MDI viewing spaces are confined to the application's window or client area . Within the client area, each document is displayed within a separate child window . MDI applications can be used for a variety of purposes - for example, working on one document while referring to another document, viewing different presentations of the same information, viewing multiple Web sites at the same time, and any task that requires multiple reference points and work areas at the same time

There are some questions your interviewer has no business asking, and this is one.  But while you may feel like answering, "none of your business, "  naturally you can't. Some interviewers ask this question on the chance you admit to something, but if not, at least they'll see how you think on your  feet. 

Some unprepared candidates, flustered by this question,  unburden themselves of guilt from their personal life or career, perhaps expressing regrets regarding a parent, spouse, child, etc. All such answers can be disastrous.

BEST ANSWER:  As with faults and weaknesses,  never confess a regret.  But don't seem as if you're stonewalling either.

Best strategy:  Say you harbor no regrets, then add a principle or habit you practice regularly for healthy human relations.

Example:  Pause for reflection, as if the question never occurred to you.   Then say, "You know, I really can't think of anything."  (Pause again, then add): "I would add that as a general management principle, I've found that the best way to avoid regrets is to avoid causing them in the first place.  I practice one habit that helps me a great deal in this regard.  At the end of each day, I mentally review the day's events and conversations to take a second look at the people and developments I'm involved with and do a doublecheck of what they're likely to be feeling.  Sometimes I'll see things that do need more follow-up, whether a pat on the back, or maybe a five minute chat in someone's office to make sure we're clear on things.whatever."