When you try to go into OEM do you get a message about the Management Server not being started? If so, you need to
create a respository via Configuration Manager,start the Management Server (via Services on NT), then sign on to OEM using sysman/oem_temp as the administrator/password. OEM prompts you to change the password at this point.
The OEM_REPOSITORY is listed under the ORADATA folder.
You can check first to see if there is an entry there before adding a second one.