OK, so I think some good progress has been made.

I am now successful with TNSPing. I've added the linked server. Now when testing the linked server, I get the following error:

A connection was successfully established with the server, but then an error occurred during the login process. (provider: Shared Memory Provider, error: 0 - No process is on the other end of the pipe.) (Microsoft SQL server, Error:233)

I AM able to launch SQL*Plus from the command prompt and run a select statement.. To do this, I'm connecting using the command:

c:\SQLPlus <user>@<servicename> where <user> and <servicename> are replaced with the correct values.

If I launch SQL*Plus from Windows, (Start>all programs>OracleHome>Application Development>SQL Plus) I am immediately prompted for the username then password, and that connection fails.

When prompted for username - if I enter <user>@<servicename>, and then the password at the password prompt, it connects.

This is progress, but I'm not sure what this is telling me.