WebClient.UploadValues doesn't work on Vista only

[Juliano.net]

I have a program that uses WebClient.UploadValues method to send login
information to a web site. It works fine on Windows XP, but on Windows
Vista the users seems to be sent wrong, because the login data is not
accepted.

Do you know if there is any incompatibility between WebClient class
and Windows Vista?

My code is like this:

NameValueCollection nvc = new NameValueCollection();

nvc.Add("UsernameControl_ID", this.User);
nvc.Add("PasswordControl_ID", this.Password);
statusMessage =
Utils.GetLocalizedString("message.status.logging_o ut");
StatusMessageChanged();

//Forcing logout
webClient.DownloadFile(Urls.Logout, Utils.HtmlTempFile);

try
{
statusMessage =
Utils.GetLocalizedString("message.status.logging_i n");
StatusMessageChanged();

byte[] response = webClient.UploadValues(Urls.Logon, nvc);

SaveToTempHtmlFile(Encoding.ASCII.GetString(respon se));

statusMessage =
Utils.GetLocalizedString("message.status.loading_p age");
StatusMessageChanged();

return UserLoggedOn();
}
catch
{
}