This is my solution for something similar to this problem:
System.Windows.Forms.WebBrowser www;
void VerificarWebSites()
{
www = new System.Windows.Forms.WebBrowser();
www.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(www_DocumentCompleted_login);
www.Navigate(new Uri("http://www.meusite.com.br"));
}
void www_DocumentCompleted_login(object sender, WebBrowserDocumentCompletedEventArgs e)
{
www.DocumentCompleted -= new WebBrowserDocumentCompletedEventHandler(www_DocumentCompleted_login);
www.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(www_DocumentCompleted_logado);
www.Document.Forms[0].All["tbx_login"].SetAttribute("value", "Gostoso");
www.Document.Forms[0].All["tbx_senha"].SetAttribute("value", "abcdef");
www.Document.GetElementById("btn_login").Focus();
www.Document.GetElementById("btn_login").InvokeMember("click");
}
void www_DocumentCompleted_logado(object sender, WebBrowserDocumentCompletedEventArgs e)
{
System.IO.StreamWriter sw = new StreamWriter("c:\\saida_teste.txt");
sw.Write(www.DocumentText);
sw.Close();
MessageBox.Show(e.Url.AbsolutePath);
}