I am not sure that you can change all of the signature responses from an IIS server. Try running a sniffer to follow the connection establishment and data transferred between the web browser and server. You can use something like Snort or TCPDump/Windump. There are many others, but I know that you can view it with these two. If I am right, then you will actually see your webserver identify itself as an IIS version whatever server.

Hope that helps.