Forcing basic authentication from desktop application

I have a C# desktop application that stores login data in a database, and uses that to automatically login to local devices that have an HTTP interface. I am using a WebBroswer control to show the web pages.

The automated login works well for form based authentication, but I'm having trouble with basic auth. I have tried to ways.

I have a helper class that passes formatted Uri and such to the browser window, which in turn passes it to the webBrowser control's Navigate method:

try{Browser winViewer =newBrowser(uri,null,null, header, deviceInfo);
            winViewer.Show();}

Here is the point inside the browser window where the navigation is done:

//pass vars and navigate to interfacetry{
            wb.Navigate(url, targetFrame, postData, headers);}

Here are the 2 ways I have tried to pass the login data to the home uri:

This one causes a COM error at runtime, with HRESULT: 0x800C000E. It seems to be a security error: StackTrace: at MS.Win32.UnsafeNativeMethods.IWebBrowser2.Navigate2

Uri uri =newUri(String.Format(@"http://{0}:{1}@{2}", targetDevice.deviceLoginName, targetDevice.devicePassword, targetDevice.ipAddressString));

The second way (and probably the correct way) I am using is to pass the encoded header directly to the browser after a 401 status code is returned. This is the method that is causing the Windows Security login popup.

Uri uri =newUri(String.Format("http://{0}", targetDevice.ipAddressString));string header ="Authorization: Basic "+Convert.ToBase64String(Encoding.ASCII.GetBytes(targetDevice .deviceLoginName +":"+ targetDevice .devicePassword ))+System.Environment.NewLine;

Can anyone tell me why the security window is popping up, and what process I can use to eliminate or hide it, so my login remains invisible?