string sUrl = "login.aspx";
HttpWebRequest oRequest = (HttpWebRequest)WebRequest.Create(sUrl);
CookieContainer oMyCookies = new CookieContainer();
oRequest.CookieContainer = oMyCookies;
// encode postdata into byte array.  the postdata string format will most likely be different and you'll have to examine the postdata going back and forth using some firefox addon like LiveHTTPHeaders
byte[] oPostData = System.Encoding.UTF8.GetBytes("username=" + HttpUtility.UrlEncode(sUser) + "&pass=" HttpUtility.UrlEncode(sPass));
using (Stream oStream = oRequest.GetRequestStream())
{
    oStream.Write(oPostData, 0, oPostData.Length);
}
HttpWebResponse oResponse = oRequest.GetResponse();
// save response cookies in our cookie object for future sessions!
foreach (Cookie oCookie in oResponse.Cookies)
{
    oMyCookies.SetCookies(sUrl, oCookie.ToString());
}
// maybe check response headers for location
string sResponseContents = null;
using (StreamReader oReader = new StreamReader(oResponse.GetResponseStream())
{
    // store server response into string
    sResponseContents = oReader.ReadToEnd();
}