The following message appears in the log files:
Could not create an instance of the counter 'XXX' (category: 'YYY')
Exception: System.UnauthorizedAccessException
Message: Access to the registry key 'Global' is denied.
Resolution:
A Sitecore application pool user has to be a member of the system "Performance Monitor Users" group to have access to the mentioned registry key. Adding the user to this group and restarting IIS should solve the problem.