Word document streamed to HTTP response causes 401 authentication demand

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Good morning all,I have a web app using Classic ASP, hosted on IIS 7.5. I need to create a word document and stream back to the client for display, however when the document is streamed into the response something (an IIS setting?) is causing a 401 error - and i cannot track it down.A colleague very kindly furnished me with some C++ code which deals with the document building & streaming and an ASP page which builds the COM objects and makes the calls etc.The document starts life as a template in an IIS virtual directory outside my ASP application. We make a copy, do some find and replace actions and then stream it into the response with the correct MIME type for the browser to handle it however it sees fit.It seems as soon as the streaming takes place (via a call to \[code\]ASPTypeLibrary::IResponsePtr piResponse->BinaryWrite()\[/code\]) the 401 response is sent backm, until that momne tthe response looks fine.The site uses Forms authentication, and i have by this point signed in, the request i see in fiddler definitely has valid session data and the rest of the site is happy with my authentication.Any ideas? (he asks with a note of desperation)p.s. I realise i haven't listed all the code, i can if it helps though...