If you have the instantclient "installed" and want to enable some diagnostic debug output, in the same folder as your tnsnames.ora
create a file sqlnet.ora
echo %TNS_ADMIN%
c:\oracle\network\admin\
file contents (remove comment to make active):
#trace_level_client = admin
#trace_unique_client = on
#trace_file_client = sqlnet.trc
#trace_directory_client = c:\\oratraces
# the last seemed particularly important to specify own directory
#adr_base=off