Networking And Gateways :: TNS Could Not Resolve Connect Identifier Specified

I have a really strange problem connecting from a new client to an Oracle server, here are the points :

- There is no problem with DB server and its listener and all that as I am able to connect from other clients
- I have installed the oraclient on this machine ( it is a XP machine ) and copy/pasted the TNSNAMES.ORA from one of those correct PCs here and when I do a TNSPING is returns OK
- even when I open oracle net manager I see the TNS entries in there and when I test a connection thru it is says successfull
- BUT WHEN I USE SQL+ or TOAD or ODBC ( any of these) trying to make a connection using the exact same TNS entries it returns :

"ORA-12154: TNS:could not resolve the connect identifier specified"

- I have tried with both IP and Host name but no luck
- TNSNames.Ora and sqlnet.ora both are located here : oracleproduct10.2.0client_1NETWORKADMIN
- there is just one oracle home
- I have already uninstalled/installed oracle client
- These lines are included in Sqlnet.ora :
SQLNET.AUTHENTICATION_SERVICES= (NTS) NAMES.DIRECTORY_PATH= (TNSNAMES)

I marked those entries in Sqlnet.ora and that made no diff in result, another thing : if I use easy connect method : sqlplus scott/tiger@<Servername>:1521/ORCL it does connect but using TNSNames still I can not get connected. so strange!!, seems somehow the TNS file even is not being picked or looked at !