Performance counters can be a great insight to how your server is performing.  The only problem is that sometimes the counters can become corrupt.  The first time I ran across this it took me a little while to find the solution.  There is a very good KB article on how to rebuild that counters that is listed below.  The easiest way I have found is to to run lodctr /R at the command prompt on the server having the issue.  You will need to have administrative rights to run the command.  This command will rebuild all of the performance counters including extensible and third-party counters in Windows Server 2003. If you are running into this problem on a cluster fully read the KB article as there are a couple extra steps.

Rebuild Performance Counters
http://support.microsoft.com/kb/300956