The default userid used by IIS is explicitly barred from having access to network resources on security grounds.

I'm not sure if it is possible to give the user IUSR_machinename, network access rights, but even if it is, you should probably talk to your SysAdmin people about it. If this is possible, and you have the appropriate rights to make this change, you should probably think carefully about why MS (not exactly known for their stringent security) chose not to give these rights by default!