improve error message if accessing the performance counters fails

src/AddIns/Analysis/Profiler/Controller/Profiler.cs

@@ -410,17 +410,26 @@ namespace ICSharpCode.Profiler.Controller
 			}
 		}
 		
+		bool usePerformanceCounters = true;
+		
 		unsafe void AddDataset(byte *ptr, TargetProcessPointer nativeStartPosition, long offset, long length, bool isFirst, TargetProcessPointer nativeRootFuncInfoPosition)
 		{
 			using (DataSet dataSet = new DataSet(this, ptr + offset, length, nativeStartPosition, nativeRootFuncInfoPosition, isFirst, is64Bit)) {
 				lock (this.dataWriter) {
 					this.dataWriter.WriteDataSet(dataSet);
 					
-					if (performanceCounterInstanceName == null)
-						performanceCounterInstanceName = PerformanceCounterDescriptor.GetProcessInstanceName(profilee.Id);
-					
-					if (performanceCounterInstanceName == null)
-						LogString("instance not found!");
+					if (usePerformanceCounters) {
+						if (performanceCounterInstanceName == null)
+							performanceCounterInstanceName = PerformanceCounterDescriptor.GetProcessInstanceName(profilee.Id);
+						
+						if (performanceCounterInstanceName == null) {
+							usePerformanceCounters = false;
+							LogString("Warning: One or more performance counters could not be accessed. " +
+							          "Please ensure that the perfmon service is enabled and running. " +
+							          "If the problem still persists try rebuilding the performance counters " +
+							          "by executing \"lodctr /R\" from a command line with administrative rights.");
+						}
+					}
 					
 					foreach (var counter in performanceCounters)
 						counter.Collect(performanceCounterInstanceName);