How to authenticate from a webpage with IIS password TROXO on IIS 6

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I have a site on IIS with a password protected with IISpassword and troxo.
When going to any of the documents within the folder windows request a password which is fine.

I have a customer area, users login via a web page. Once logged in they should be able to view any of the documents.

Does any one know how I can programatically login users to the protected folder, for instance, is it using a session etc

Many thanks