I used to interview college candidates for Microsoft quite often, but haven't done that in a while, so my advice might be dated. But here's what I remember: there's an emphasis on "intellectual horsepower," or the ability to solve problems dynamically. And of course on coding. Here's what I'd advise:

- Speak your thoughts out loud as you reason through problems. Don't just spend 5 silent minutes staring at the whiteboard writing random things. Even when you're writing code, describe what you're thinking.

- Get code correct before you start optimizing to get it fast. And let the interviewer know that it's what you're doing.

- Test your own code first before declaring yourself done. It is far better to find your own bugs than to have the interviewer find one.

Best wishes! Let me know how it goes!