This is your opportunity to let yourself shine during the interview. Don't pick something easy like the simpleton pattern, think about a couple of more complex examples that the interviewer probably hasn't heard recently. This will make you stand out a bit more.
Here are some ideas that you should be talking about, Simplicity, Readability, Modularity, Layering, Design, Efficiency, Clarity and Elegance. Check this Microsoft MSDN article for those who need to study these ideas further
You may be asked to use a whiteboard. If you aren't and there is one available, take advantage of it. You may be asked to choose a project yourself, but also be prepared for the interviewer to choose one at random and ask you to explain it. It is a good idea to quickly go through your CV before the interview, to refresh your mind on what is in there.
Here is a link to questions asked during interviews for Microsoft positions. Other Microsoft interview information, including links to Microsoft recruiter blogs can be found on the same website.