More than often I have found that when engineers have installed VMware vcenter 5, they have tended to use the same password for the database and the admin@system-domain password. Although this is not good practice it helps to find out the admin@system-domain password as did you know that the database password is kept on the server in plain text!!!
Browse to the path below and open the config.properties file in notepad.
Gives you a good place to start anyway.