I recieved e-mail form yahoo/microsoft UK that I won a sweepstakes. Is this a scam?

They have been e-mailing me everyday. I did answer them because the had a letterhead with the microsoft building and the microsoft, yahoo, msn logo. Now their courier company e-mailied me and requesting from 500.00 to 1000.00+ to sent me $850,00.00 US $'s. What do I need to do now?