Auto network credentials

Hi!
I'm writing a program that reads some data from a website to do some calculations and display the results on the GUI.
The program works correctly without a proxy, but unfortunately it is going to be used mostly behind one.

The computers where the program will be installed use network accounts (in a domain) and after opening the browser the window's account username/password combination is valid to surf.

My problem is that I'm unable to do the same, that is get the cached username/password combo to connect to internet (using HttpWebRequest) without asking the user to type the required information (I tried to use NetworkCredentials, but the program still returns a 407 Authorization required error).

How can I do that?

Ok, it seems that what I want to do is not possible.
The reason is security of information, it would be quite easy to steal login information from a user if the password was available.

I ended up using a small form that pulls up the domain and username with

PHP Code:

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WindowsIdentity wi = System.Security.Principal.WindowsIdentity.GetCurrent(); 


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and than asks for the password.

I currently check if the login information is correct with this:

PHP Code:

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try
{
   HttpWebRequest proxyTestRequest = (HttpWebRequest)WebRequest.Create(destWebPage);
   WebProxy testProxy = new WebProxy();
   testProxy.Address = proxy.GetProxy(destWebPage);
   testProxy.Credentials = new NetworkCredential(proxyUser, proxyPassword, proxyDomain);
   proxyTestRequest.Proxy = testProxy;
   WebResponse myResponse = proxyTestRequest.GetResponse();
   myResponse.Close();
}
catch (WebException we)
{
   if (we.Status == WebExceptionStatus.RequestProhibitedByProxy)
   {
       //Rest of the code that opens a form as dialog, gets the login combination and stores it as proxyDomain, proxyUsername and proxyPassword
   }
} 


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Is it a good way to do it or I should use something different? In this case what should I use?