I don't know how far this would help, try the following on your server:

Go to ORACLE_HOME/network/admin and make sure the tnsnames.ora file had the correct settings.

Now try to connect to your database from the DOS prompt as

sqlplus system/manager@Oracle816

If it works fine

Then try to give the

sqlplus system/[email protected]

Then try opening your SQLTEXT and enter your service name under the host on the prompt window. If that doesn't work then try giving the ORCL81.WORLD and see whether it connects.

Sam