One more thing I want you to verify is if you have installed two different versions of Oracle (or Oracle Agents) on that server.

For example on one of my NT servers I have two versions installed (8.0 and 8.1.5)
They both have their own agents and their own sets of listener etc files.

From OEM when I try to monitor 8.0 databases I need to make sure that the 8.0 agent is running. It does not work with the 8.1.5 agent.

- Rajeev