Implement Forms Authentication through Windows authentication

Hi everyone.  We have an internet portal using .NET 1.1 that uses forms authentication, so users go to login page (a webpart in the default page) and enter their credentials.  Now bosses want also that the portal to be logged in the intranet, working like this:
If a user is accessing the portal outside the intranet (in the internet), the portal should use the normal Forms Authentication.If a user is accessing the portal inside the intranet (with their active directory credentials), the portal should automatically log the user in the website (set the forms auth cookie, etc).For this purpose, a new table was added that maps the UserID's from the portal, to logins from active directory.I'm having a rough time here.  What I initially did was a HttpModule that adds a AuthenticateRequest method, checking the Request.ServerVariables["LOGON_USER"] value, then validates against the mapping table and doing log ...