You don't happen to have more than one Oracle home by any chance?
No. There is only one oracle home
Are you selecting the tns service name from the drop down in the Oracle ODBC driver configuration dialog?
Yes I have chosen my service name from drop down list.
I have fixed this issue by adding the following lines to "sqlnet.ora" file which exist under "C:\oracle\product\10.2.0\client_1\NETWORK\ADMIN"
names.default_domain = world
note: world represent extension for your instance name (ex: x.world where x is your TNS name)