Install all required files for DNSDiag
Tools such as DNSDiag require multiple system dlls.
The fastest/best way to install all the required files is to go to "Add Remove Programs", select "Add Windows Components", and then drill down into "Application Server\Internet Information Serivces (IIS)" and install the "Common Files" option.
Then add C:\Windows\System32\inetsrv to your %Path% variable.