This does not work because the clients do not have access to the server C. It works in development because your client is on the same LAN. What you need to do is to stream the file to client from the IIS server. Here is an example to do this in ASP.NET:
Response.WriteFile(@"\\C\documents\document.doc");
Of course, a full solution will be more complex as you need to put the MIME Type of the file in the header.
Here is another article that describes the process:
http://support.microsoft.com/kb/306654/[
^]
Submit an article or tip