Uri url = new Uri("http://www.yoursite.com/login.aspx");
WebRequest r = WebRequest.Create(url);
r.Method = "POST";
string postData = "txtUsername=YOUR_USERNAME&txtPassword=YOUR_PASSWORD";
byte[] data = Encoding.UTF8.GetBytes(postData);
r.ContentLength = data.Length;
r.ContentType = "application/x-www-form-urlencoded";
using (System.IO.Stream s = r.GetRequestStream())
{
    s.Write(data, 0, data.Length);
}
string html = string.Empty;        
using (WebResponse response = r.GetResponse())
{
    using (Stream stream = response.GetResponseStream())
    {
        using (StreamReader sr = new StreamReader(stream))
        {
            html = sr.ReadToEnd();
        }
    }
}