There are several things that can cause this problem. The first thing to make sure of is that you have the Oracle 8 client installed. If you can use the Oracle 8 client tools (such as SQL Plus) to successfully connect, then you may have a path issue. The easiest way to test this is to copy the SQL Plus EXE from the Oracle bin directory to your desktop and see if you can run it. The SQL Plus EXE is typically in your c:\ORANT\BIN or c:\ORA95\BIN directory (depending on the OS, and assuming it is installed on the C drive). If you try to run it from the desktop and you get an error about it not finding a DLL, then you have a path issue.

The way to fix a path issue is to right click on the file c:\autoexec.bat and select Edit. Look for the line that starts with something like:

set PATH=C:\....

For Windows NT:

set PATH=C:\ORANT\BIN;

For Windows 95/98:

set PATH=C:\ORA95\BIN;