If you have an offer that you ...
If you have an offer that you have accepted, and you tell Microsoft that, they will ask you for contact details of the HR of the company whose offer you signed to contact them and ask them if its ok for Microsoft to interview you, if you dont give them details they will cancel your interview.
Microsoft has this policy where they dont consider you as a candidate if you have signed an offer already (and they know this, you can just as easily hide it) and will ask you to re apply after 8 months.
Please be careful of what details you reveal. My personal experience, I went on site and just to come back without being interviewed because I told them I have signed an offer and the only reason why I am attending the interview is because Microsoft is the better option.