How to obtain user credentials from Active Directory via Web Service

Hello to all, I am currently developing a web application in c# that requires a web service that must obtain the Active Directory user credentials (login, password). This application will be used solely in an intranet and those users that connect through internet must use a VPN (Virtual Private Network). All users will be connected through Windows Authentication. I have made some research upon this subject and have found basically no help. Can anybody suggest how this can be done and how can it be developed?