most likley, .dll is an ISAPI Server Extension DLL written in C++ or other languages
see
Overview: Creating an Internet Server Extension (ISAPI) or Filter DLL
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/vccore98/HTML/_core_overview.3a_.creating_an_internet_server_extension_.28.isapi.29_.or_filter_dll.asp
you don't need it in ASP.NET, you can write custom HttpHandlers