IIS (Microsoft / Windows Server HTTP server) supports a technology rival to CGI called ISAPI, which was deployed using .dll files. .dll is a Microsofts' dynamic library standard, commonly produced by e.g. C/C++, but also by .Net.

The thing that stands out the most if the fact that most older IIS setups did not use rewrite on URLs, so instead of pointing to a nice looking path, they were pointing to .aspx files or .dll.