This could be due to two different things:
- the listener isn't running (or isn't running on the port you think it is)
- the tnsnames.ora entry you're using isn't referencing the host and port where the listener is running
If you have a standalone computer with no network attached, then you may try setting an environment variable (ORACLE_SID) to the name of your Oracle database instance. Then, you should be able to connect without using a connect string since the ORACLE_SID variable will be used by default.
This was first published in November 2004