How to get the roles for a user other than the current one

[C# Programming]

I am writing a web application that I want to use windows authentication for. The only problem is that the users using it may not always be running it from there own machine. I want to check the roles for the current windows user and if they have the required role take them directly into the application. If the current user does not have the required role I will take them to a sign-on page so they can enter their windows user i.d. and password. Then I will check if that user has the required role. The problem is that I can only find a way to check the roles for the current windows user.

Is there a way to validate the user that signs in to the application rather than the windows user and check the roles or groups they are in?

Thank you
Marty