I know there was a thread just 2 days back about this but I have slightly different problem with tnsnames.ora file.
I have a test server oracle816 here which has 8i database.
When I searched for tnsnames.ora file I found it in c:\oracle\ora81\network, c:\oracle\ora81\network\admin,
c:\orant\net80\admin, c:\orant\net80\tnsapi, c:\orant\network\admin.

This machine is NT machine.

I changed all these tnsnames.ora file on this server and on my machine workstation to read...

(ADDRESS = (PROTOCOL = TCP)(HOST = Oracle816)(PORT = 1521))

I am confused because using system/manager.. orcl81 to sql plus or sql worksheet or any other utility, I get error.
I can login from my workstation NT to this server NT. But when I try to log on from this server where this database is I get error saying TNS could not resolve service name.
But when I try to log in as system/manager and with blank service name on this server it works !!! Why it does not take that service name ? What's wrong ? which tnsnames.ora is it looking at ?