If you've researched M$ interviews before, you know that this will be asked almost definitely. The answer to this is important. Are you looking to get in at any big company, or do you genuinely care about Microsoft as a whole? What of their products excite you, and how much thought have you given this? What about the people? Do you just want into Microsoft, or do you care about the position you're interviewing for, too? Give this lots of thought. Ponder it and even practice answering the question out loud.