For some versions of Windows Server 2008 and Windows Vista, the Avg Read and Write Time disk metrics are reported as 0. This is due to a known Microsoft Windows bug, please see Microsoft Knowledge Base. If this would impact you, you can download the hotfix available at that link. This refers to the following metrics:
Hardware Resources|Disks|<mount_point>|Avg Read Time (ms)
Hardware Resources|Disks|<mount_point>|Avg Write Time (ms)