Can you run SQLPLUS from Windows without a TNSNAMES.ora entry? Oracle 11g comes bundled with SQL Developer (just like OWB is part of the Oracle 11g database typical install). actually 11g is much more application centric and even the default folder for install is called apps.
Anyways, try out the SQL Developer that is bundled with the 11g RDBS install. It uses JDBC to connect to the database. You can run SQLPLUS as well from SQL Developer by pointing to the SQLPLUS.exe as the external executable and it will use the JDBC information to connect to the external database (even a 10G).