Last week I tried to install Oracle Forms 11g. This install created a new Oracle_Home on my computer, which resulted in my database not starting up anymore.
The cause of this was, that my computer did not pick up the right tnsnames.ora file anymore, but instead it picked up the new tnsnames.ora that was created during my Forms installation.
To avoid having to have multiple copies of the same tnsnames.ora file on your computer, you can create a registry key in all Oracle_Homes. This will force your computer to pick up the same file for all your Oracle_Homes and thus allow you to have just a single copy of tnsnames.ora to manage.
To do this, start the Windows Registry editor by using Start -> Run -> Regedit and find your Oracle_Homes under HKEY_LOCAL_MACHINE -> SOFTWARE -> ORACLE and rightclick on your Oracle_Home entry. Choose New -> String Value and enter the required information similar to the following screenshot (directory on harddrive may differ on your computer).
The registry key is called TNS_ADMIN and it’s value is the location of your tnsnames.ora file on the harddrive.
But remember to only add this key to the Oracle_Homes that do not contain your central tnsnames.ora.