Sounds like a perennial Oracle problem. Each version of Oracle client tools wants its own compatible version of Sqlnet, Net8, etc. Do a file Find on Tnsnames.ora. You will undoubtedly will have multiple copies of your Tnsnames.ora. Check also your Sqlnet.ora file, it should specify Tnsnames if that is your environment.
It becomes rather dicey on your client, if that is were you are doing your work. Your Windows path and registry can be come a horror. You may want to edit your path on a client to reflect that one particular Oracle\bin is before another. Another related problem is that various versions of your tools will have different executables, depending on the version of Oracle. SqlNet v2.3 has Tnsping, but in Net80 it is called Tnsping80, which is again renamed back to Tnsping in 8i. So, depending on your path, your call on Tnsping may be a different executable than what you really wanted. To futher enliven this mix, some Oracle tools that are called "6i" may actually be using an older version of SqlNet, so that may have to be looked into.
Hope this helps. Also be aware that some Oracle products will automatically over-write an existing Glogin.Sql, Tnsnames.ora and Sqlnet.ora files. Saves these prior to loading any Oracle client tool.