object o = null;
object o1 = System.Text.Encoding.ASCII.GetBytes("product=mail&u=dsfghfgh&psw=sdf3&登录=UrlDecode('登录')");
object o2 = "Content-Type: application/x-www-form-urlencoded\r\n"; //必须要加的
axWebBrowser1.Navigate("http://mail.sina.com.cn/cgi-bin/login.cgi", ref o, ref o, ref o1, ref o2);