WebClient.Uploa dValues doesn't work on Vista only

[Juliano.net]

I have a program that uses WebClient.Uploa dValues 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:

NameValueCollec tion nvc = new NameValueCollec tion();

nvc.Add("Userna meControl_ID", this.User);
nvc.Add("Passwo rdControl_ID", this.Password);
statusMessage =
Utils.GetLocali zedString("mess age.status.logg ing_out");
StatusMessageCh anged();

//Forcing logout
webClient.Downl oadFile(Urls.Lo gout, Utils.HtmlTempF ile);

try
{
statusMessage =
Utils.GetLocali zedString("mess age.status.logg ing_in");
StatusMessageCh anged();

byte[] response = webClient.Uploa dValues(Urls.Lo gon, nvc);

SaveToTempHtmlF ile(Encoding.AS CII.GetString(r esponse));

statusMessage =
Utils.GetLocali zedString("mess age.status.load ing_page");
StatusMessageCh anged();

return UserLoggedOn();
}
catch
{
}