Through reading these articles I installed Fiddler, and watched my transaction. Something about the hostname looked odd to me so I used IP address instead. Code now works. The hostname of course works when you put it in a browser, but not in my code.
"http://sp1" is a browsable sharepoint server, but this line doesn't like it:
ClientContext clientContext = new ClientContext(comboBox1.Text);
I am having my server team put in a nice DNS entry for the server.