If you know the URL of the file you're trying to download, you can use the WebClient to do the login and the download. Just set the WebClient credentials and call its DownloadFile() method....
WebClient webClient = new WebClient();
webClient.Credentials = new System.Net.NetworkCredential("UserName", "Password", "Domain");
webClient.DownloadFile("fromURL", "C:\localfile.txt");
This is untested, by you may still be able to use your WebBrowser object to login, and then use a WebClient with the default credentials and call its DownloadFile() method:
WebClient webClient = new WebClient();
webClient.UseDefaultCredentials = true;
webClient.DownloadFile("fromURL", "C:\localfile.txt");
OR you can try adding the OnFileDownload event. If it's called prior to the Save File dialog, then you can cancel the download, and launch the download through the WebClient object....