Ok, I looked into the results 0 and 631. I was clicking cancel in the DUN dialogue while testing (because the appearance of the dialogue was enough to confirm for me that my WININET call was working). So clicking CANCEL returns the error code 631. If I dial and allow DUN to complete the connection, it returns 0... clearly 0 is showing success. So I agree with you on the error codes.

What this means for the workstation that is not dialing, is that WININET.DLL is saying "Ok, command successful, it worked!" when in actual fact nothing happened. I will look into the Internet Options - Connections options to see if the problem is there...

Thanks for your help.