With the additonal information:
"Unable to find SQL Server Process ID <ProcessId> on server <Server>
(Microsoft.SQLServer.Management.ResourceMonitoring)"
The OS of the Machine is Windows 2003 R2.
ARITHAbort is not on by default.
The machine was recently virtualised and is running on an ESX 3.5 host.
Activity Monitor works on our other SQL Server 2008 Standard Edition
Instnces on other Machines (Windows 2003 R2/Windows 2008 R1).
Any assistance would be much appreciated.