If it's not in the GAC or Registry, then there's likely no way for the .NET Framework to tell where the DLL is. In fact, EVERY COM component needs to be registered to work, since it uses the GUID in the registry to find it.

You need to deal with COM Interop. What you want for your Assembly is a COM Callable Wrappter (CCM):
http://msdn2.microsoft.com/en-us/library/f07c8z1c.aspx

Read this article and the related ones to find out what you can and can't do.