When monitoring Windows Server 2008 using the Server Analysis Module it is possible for the System 5 access denied error to be thrown despite specifying the correct credentials.  This is caused by the increased security present in Windows Server 2008 which detects that the machine doing the monitoring (on which Silk Performer is installed) is running under a different user account than the credentials being passed by Performance Explorer.  

To resolve this issue right click on Performance Explorer icon and choose run as - now specify the same credentials which would normally be entered in the connection parameters dialog to connect to the server to be monitored.  It is not enough to provide administrator credentials for the Windows 2008 Server, the same administrator credentials need to be used to invoke Performance Explorer for this to work since Windows 2008 requires elevation of privileges.  Another alternative would be to log into the Performance Explorer machine with an account with privileges to monitor the Windows 2008 Server.