Authentication problem

I have an existing application that uses web services. We will be adding web pages in the near future. We have many clients using our existing windows/web services product. We Active Directory for the users but each customer has a separate database, so we store the database as part of the username. So in AD, the user is like this: domain\database.user
The user needs to login like:
user: bob
password: *****
domain: database
What is the best way to accomplish this? In windows we do this in code so its not a problem. Once the user logs in, I will need to redirect them to a version specific website as well, but that is less of a problem.
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