A colleague of mine came across a situation where SCOM performance reports were not working due to corrupt performance counters on a particular server. This situation can be easily resolved by rebuilding the performance counters using the lengthly procedure outlined at http://support.microsoft.com/kb/300956/en-us
In PerfDataSource, could not resolve counter LogicalDisk, Free Megabytes, C:. Module will be unloaded.
One or more workflows were affected by this.
The short version:
On the affected server open an elevated command prompt
Navigate to c:\windows\system32
Type lodctr /R and press enter
Shortly you should see the following message:
Info: Successfully rebuilt performance counter setting from system backup store.