function ajaxLogin()
{
var headnavi = document.getElementById("headnavi");	
var constr='';
constr += '<form name="user_info">';
constr += '用户名：<input class=hinput  type=text name=username  />';
constr += ' 密码：<input class=hinput type=password name=userpass />';
constr += '<input class=hsubmit type=button value=登陆 onClick=saveUserInfo()>';
constr += '</form>';
headnavi.innerHTML=constr;
}
function saveUserInfo()
{
var msg = document.getElementById("msg");
var f = document.user_info;
var username = f.username.value;
var userpass = f.userpass.value;
var filter=/^\s*[A-Za-z0-9_]{2,16}\s*$/;
  if(username=="")
  {
    alert("用户名密码不能为空！");
    return false;

  }
  if(!filter.test(username))
  {
    alert("用户名不合法，必须是2到16位字母数字或下划线！");
    return false;

  }
  if(userpass=="")
  {
    alert("密码不能为空！");
    return false;
  }
var url = "/ajax_output.php";
var postStr      = "username="+ username +"&userpass="+ userpass;
msg.innerHTML = "正在登陆，请稍后...";
  var ajax = false;

      if(window.XMLHttpRequest) 
	  	   { 
           ajax = new XMLHttpRequest();
           if (ajax.overrideMimeType)
		     {
              ajax.overrideMimeType("text/xml");
             }
            }
      else if (window.ActiveXObject) {
           try {
            ajax = new ActiveXObject("Msxml2.XMLHTTP");
             } 
			catch (e) 
				{
                     try {
                           ajax = new ActiveXObject("Microsoft.XMLHTTP");
                          } catch (e) {}
                    }
           		 }
            if (!ajax) { 
                    window.alert("不能创建XMLHttpRequest对象实例.");
                    return false;
            }
ajax.open("POST", url, true);
ajax.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
ajax.send(postStr);
ajax.onreadystatechange = function() { 
      if (ajax.readyState == 4 && ajax.status == 200) { 
       msg.innerHTML = ajax.responseText; 
      } 
} 
}