Some base values are not updated when you use the Win32_PerfRawData_PerfDisk_LogicalDisk WMI class to calculate performance data in Windows Vista or in Windows Server 2008
On a computer that is running Windows Vista or Windows Server 2008, you use some raw performance classes of Windows Management Instrumentation (WMI) to calculate performance data. However, you receive incorrect data from the performance classes.
For example, you calculate some performance data from the AvgDiskSecPerTransfer and AvgDiskBytesPerTransfer_Base values in the Win32_PerfRawData_PerfDisk_LogicalDisk class. However, the AvgDiskBytesPerTransfer_Base value is not updated as expected.
This problem does not occur when you use certain formatted performance classes, such as the Win32_PerfFormattedData_PerfDisk_LogicalDisk class.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB961435.