The MDSN samples include a C++ project for building a ISAPI DLL which performs authentication for web resources against a text file. This project, AuthFilt, is one of the samples supplied with the .NET Platform SDK, available from here. Once you've installed the SDK, the sample code is located in the AuthFilt folder at Program Files\Microsoft Platform SDK\Samples\Web\iis\filters.
The filter is something I want to look at because it could be adapted to provide a readily customisable means of securing web folders on Windows. So I need to check it works ok, and whether or not I'll be able to modify it to our needs.
During the investigatory phase, I'll be using Visual C++ express edition, available from this link.