function DoCallback(data)
{
    // branch for native XMLHttpRequest object
    if (window.XMLHttpRequest) {
        req = new XMLHttpRequest();
        req.onreadystatechange = processReqChange;
        req.open('POST', url, true);
        req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        req.send(data);
    // branch for IE/Windows ActiveX version
    } else if (window.ActiveXObject) {
        req = new ActiveXObject('Microsoft.XMLHTTP')
        if (req) {
            req.onreadystatechange = processReqChange;
            req.open('POST', url, true);
            req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
            req.send(data);
        }
    }
}

function processReqChange() {
    // only if req shows 'loaded'
    if (req.readyState == 4) {
        // only if 'OK'
        if (req.status == 200) {
            eval(what);
        } else {
            alert('There was a problem retrieving the XML data: ' +
                req.responseText);
        }
    }
}

            var url = "login.php";
            var what = "LoginStatus(req.responseText)";

            function CheckLogin()
            {
                var username = document.getElementById("username").value;
                var password = document.getElementById("password").value;
                var captcha = document.getElementById("captcha").value;

                DoCallback("username="+username+"&password="+password+"&captcha="+captcha);
            }

            function LoginStatus(status)
            {
            	//alert(status);
                if(status == 0){
                    //alert("Bad login!");
                    toggleBox('textDiv',0);
                    toggleBox('formDiv',1);
                    location.reload()
               } else{
                    //alert("Login OK!");
                    toggleBox('formDiv',0);
                    toggleBox('textDiv',1);
					document.getElementById('textDiv').innerHTML="Bienvenido " + status;
                  }
            }
            
function toggleBox(szDivID, iState) // 1 visible, 0 hidden
{
    if(document.layers)	   //NN4+
    {
       document.layers[szDivID].visibility = iState ? "show" : "hide";
    }
    else if(document.getElementById)	  //gecko(NN6) + IE 5+
    {
        var obj = document.getElementById(szDivID);
        obj.style.visibility = iState ? "visible" : "hidden";
    }
    else if(document.all)	// IE 4
    {
        document.all[szDivID].style.visibility = iState ? "visible" : "hidden";
    }
}

function IsNumeric(form)
   //  check for valid numeric strings	
   {
   var strValidChars = "0123456789.-";
   var strChar;
   var blnResult = true;
	var strString = form.username.value;
   if (strString.length == 0) return false;

   //  test strString consists of valid characters listed above
   for (i = 0; i < strString.length && blnResult == true; i++)
      {
      strChar = strString.charAt(i);
      if (strValidChars.indexOf(strChar) == -1)
         {
         blnResult = false;
         }
      }
   //return blnResult;
   if(!blnResult){
   	alert("El campo identificador ha de ser númerico");
   	form.username.value="";
   	form.username.focus();
   }
}