var oXmlHttp = null;
  var iTimeoutId = null;
  
  //function to validate fields
  function validateField(oEvent) {
      oEvent = oEvent || window.event;
      var txtField = oEvent.target || oEvent.srcElement;
      
      var btnNext = document.getElementById("btnNext");
      btnNext.disabled = true;    
      
      if (iTimeoutId != null) {
          clearTimeout(iTimeoutId);
          iTimeoutId = null;
      }
      
      if (!oXmlHttp) {
          oXmlHttp = zXmlHttp.createRequest();
      } else if (oXmlHttp.readyState != 0) {
          oXmlHttp.abort();
      }
      
      oXmlHttp.open("get", "ValidateForm.php?" + txtField.name + "=" + encodeURIComponent(txtField.value), true);
      oXmlHttp.onreadystatechange = function () {               
          
          if (oXmlHttp.readyState == 4) {
              if (oXmlHttp.status == 200) {
                  var arrInfo = oXmlHttp.responseText.split("||");
                  var imgError = document.getElementById("img" + txtField.id.substring(3) + "Error");
                  
                  if (!eval(arrInfo[0])) {
                      imgError.title = arrInfo[1];
                      imgError.style.display = "";
                      txtField.valid = false;                    
                  } else {
                      imgError.style.display = "none";
                      txtField.valid = true;
                  }
                  
                  btnNext.disabled = !txtField.valid;
              } else {
                  alert("An error occurred while trying to contact the server.");
              }
          }
      };
      
      iTimeoutId = setTimeout(function () {
          oXmlHttp.send(null);
      }, 500);
  };
  
  //if Ajax is enabled, disable the submit button and assign event handlers
  window.onload = function () {
      if (zXmlHttp.isSupported()) {
          var btnNext = document.getElementById("btnNext");        
          var txtUsername = document.getElementById("txtUsername");
  
          btnNext.disabled = true;
          txtUsername.onkeyup = validateField;
          txtUsername.onchange = validateField;
          txtUsername.valid = false;        
      }
  };