Today we upgraded our main application from Windows 2000 to Windows 2003. One of the problems was the more restrictive settings of IIS in Windows 2003. Our problem was that IIS would no longer serve .dtd documents. We had to explicitly tell IIS that .dtd and .ent files were ok to process.
Here's how to do it:
Open IIS Management Console
Right Click on your virtual directory
Select "HTTP Headers"
Select "MIME types"
Set the extension to ".dtd"
and application to "application/octet-stream"