This can occur if certain performance counters are no longer registered as WMI Classes. In order to reregister all counters, simply perform the following steps on the remote system being monitored:
1. Stop the Windows Management Instrumentation service.
2. Open a CMD prompt
3. Type one of the following commands, depending on the operating system:
For Windows 2003 or later: wmiadap.exe /f
For Windows 2000 or previous: winmgmt /resyncperf
4. Press Enter
For more information on wmiadap, see http://support.microsoft.com/kb/820847