Sharing the system folder on several domains

[eluser]Unknown[/eluser]
I have tried to share the system folder on my webserver so it can be reached on several domains connected to my webserver, but I get the following error:

Your system folder path does not appear to be set correctly. Please open the following file and correct this: index.php

It is a clean installation and I have created the "system" folder as a Virtual Directory on my ISS manager pointed to an folder where I have copied the System files and folders into.

What is wrong?

systeminfo:
CI version 2
Windows Server 2008 enterprise
IIS manager 7