System Process At 100% Processor Time

We have a server where the system process is pegging the processor 95-100% usage. Reboots don’t resolve the issue. We have stopped every service that can be stopped without killing the server and have not been able to get the processor usage to drop. You can still open
applications with no issues or delay. The fact that the processor usage is so high does not appear to be affecting server performance at all. The server is a DC, and is running Exchange and Backup Exec. It is also runningNorton Antivirus Corporate Edition. Any ideas on additional tests that we can run to get to the root of the problem? Is there anything out there that typically causes this type of problem?

Any help on this would be greatly appreciated.

Thanks!

Jason