Exception: System.UnauthorizedAccessException Message: Access to the registry key 'Global' is denied. Source: mscorlib

To add the user a different way:

Go to the Start menu in Windows 7.
Type "Edit Users" in the search box.
Open "Edit Local Users and Groups".
Go to the groups folder.
Right-click on the "Performance Monitor Users" group and go to properties.
Click the Add... button.
Type in "IIS APPPool\DefaultAppPool" and click Check Names.
Click OK and you should be good to go.

Rather than the net localgroup "Performance Monitor Users" defaultapppool /add