To force your PC to use only one TNSNAMES.ORA, setup the TNS_ADMIN environment
variable to point to the directory where all Oracle products should search for
the TNSNAMES.ORA and SQLNET.ORA files.

Perform the following steps to setup the TNS_ADMIN variable:

1. Open the 'System Properties' dialog box:

a. Click on the START button in the O/S toolbar.
b. Select Settings -> Control Panel
c. Double click on 'System' icon.

2. Set up the TNS_ADMIN variable:

a. In the 'System properties' dialog box, click on tab 'Environment'
b. Highlight OS in the list of Systems Variables.
Now, on in the bottom left corner you should see Variable:OS and
Value:Windows_NT
c. Change the Variable from OS to TNS_ADMIN
d. Change the Value from Windows_NT to the complete path were the
configuration files (TNSNAMES.ORA and SQLNET.ORA) will reside.
Example: d:\orant\net80\admin
e. Click on the 'Set' button.
f. Click Apply.
In the System Variable table you should now see the TNS_ADMIN variable
with the path to the location of the configuration files.
g. Click OK to close window.
h. Close the Control Panel.
i. It is necessary to log out and back in again for this to take effect.

Note: you can define System and User Variables;
- System environment variables are the same no matter who is logged
on to the computer. You can change values or add new variables when
you are logged on as a member of the Administrator group.
- User environment variables are specific to the connected user.

Reference
---------

"Oracle Designer Release Notes, Release 2.1.2 - Production",
November 1998 (Z23509-03), Topic 'SQL*Net, SQL*Plus and the
Import/Export Utilities'