How can I enable Enterprise Manager in Oracle 10g?

Question

Hi,
We recently installed Oracle 10g on a MS Window server and after the installation which we created 2 instances each with a default schema from an existing data file (.dmp). After the installation, I realized that somehow, the DBConsole entry is missing in the Windows Services which then prevent me from being able to launch the new browser-based EM.
We thought we have picked all the components during the installation but for some reasons, it seems like the necessary files or configurations for EM was not there. So I was wondering if anyone know how I can manually install and configure EM?
Initially I thought if I go thru Oracle Universal installer and re-install everything again, it might help but I am concern that it will then wipe out the 2 instances and schemas that we have already created, especially since I have also applied the 10.2.0.3 patch already. So there was some discussions on calling the emctl program which when I tried, it I got error saying that ORACLE_SID not found. Then there was also some documentation on using the EMCA command to build the dbconsole, is that what I should do?
Lastly, I was under the impression that one will only setup one EM per server, which means in a perfect world, I will have one instance of the EM which will then allow me to manage the multiple instances (and their schemas) from the same console, is that correct? Or is one EM instance only manages one instance of the database? So if I have TESTDV & TESTUT configured on the same server, I will need to also create two EM consoles?
I am an amature and am more familiar with Oracle 9i which the EM seemed to be easier to use and setup. So I think I must have skipped or missed some steps during my 10g installation but am concerned that if I reinstall, I will lose everything.
Thank you very much.

Answer 1

First of all, thank you as always for taking the time...
I apologize in advance if I am not using the right terminology. Basically, I have only one Oracle Home directory on that server but I am able to create two different instances and in both cases, I have left the port number to be the same thinking that it's only one physical installation. I thought I would attach my TNSNAME and LISTNER ora files below to give you a better picture of my current setup.
So for me to connect to the database and the appropriate instance for example, like using SQL Developer, all I have to do is specify the Host Name to DBServer and then the Service Name to either TESTDV or TESTUT. Then I can connect to each of the instance directly.
But I only have one Oracle_Home directory and use the same port number as the default choice. So to your comment, if I only have one Oracle_HOME and one Port number, should I only install one EM instance?
Thanks much.
******In the TNSNAMES.ORA file:*****
EXTPROC_CONNECTION_DATA =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC0))
(CONNECT_DATA =
(SID = PLSExtProc)
(PRESENTATION = RO)
TESTDV =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = DBSERVER)(PORT = 1521))
(CONNECT_DATA =
(SERVICE_NAME = TESTDV)
TESTUT =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = TCP)(HOST = DBSERVER)(PORT = 1521))
(CONNECT_DATA =
(SERVICE_NAME = TESTUT)
*****In the LISTNER.ORA file:*****
SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(SID_NAME = PLSExtProc)
(ORACLE_HOME = C:\oracle\product\10.2.0\db_1)
(PROGRAM = extproc)
(SID_DESC =
(GLOBAL_NAME = TESTDV)
(ORACLE_HOME = C:\oracle\product\10.2.0\db_1)
(SID_NAME = TESTDV)
(SID_DESC =
(GLOBAL_NAME = TESTUT)
(ORACLE_HOME = C:\oracle\product\10.2.0\db_1)
(SID_NAME = TESTUT)
LISTENER =
(DESCRIPTION_LIST =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = DBSERVER)(PORT = 1521))
(ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC0))
)