We get this error only in one of our users PC with office 2003 we tried re Reinstalling the Office 2003 Primary InterOp Assemblies also. Why should this happen in only one users desktop when we have this add-in running on 100 of other workstation ? Microsoft.Office.Interop.Outlook.Application oApplication = null; try { comboBox_Folder.Items.Clear(); Type oType = Type.GetTypeFromProgID("Outlook.Application"); if (oType != null) oApplication = (Microsoft.Office.Interop.Outlook.Application)Activator.CreateInstance(oType); Unable to cast object of type 'System.__ComObject' to type 'Microsoft.Office.Interop.Outlook.ApplicationClass'. at System.Runtime.Remoting.RemotingServices.AllocateUninitializedObject(RuntimeType objectType) at System.Runtime.Remoting.RemotingServices.AllocateUninitializedObject(Type objectType) at System.Runtime.Remoting.Activation.ActivationServices.CreateInstance(Type serverType) at System.Runtime.Remoting.Activation.ActivationServices.IsCurrentContextOK(Type serverType, Object[] props, Boolean bNewObj) at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandle& ctor, Boolean& bNeedSecurityCheck) at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean fillCache) at System.RuntimeType.CreateInstanceImpl(Boolean publicOnly, Boolean skipVisibilityChecks, Boolean fillCache) at System.Activator.CreateInstance(Type type, Boolean nonPublic) TIA John