//function to validate fields
  function validateField(oEvent) {
      oEvent = oEvent || window.event;
      var txtField = oEvent.target || oEvent.srcElement;
      var oXmlHttp = zXmlHttp.createRequest();
      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");
                  var btnSignUp = document.getElementById("btnSignUp");
                  
                  if (!eval(arrInfo[0])) {
                      imgError.title = arrInfo[1];
                      imgError.style.display = "";
                      txtField.valid = false;                    
                  } else {
                      imgError.style.display = "none";
                      txtField.valid = true;
                  }
                  
                  btnSignUp.disabled = !isFormValid();
              } else {
                  alert("An error occurred while trying to contact the server.");
              }
          }
      };
      oXmlHttp.send(null);
  };
  
  function isFormValid() {
      var frmMain = document.forms[0];
      var blnValid = true;
  
      for (var i=0; i < frmMain.elements.length; i++) {        
          if (typeof frmMain.elements[i].valid == "boolean") {
              blnValid = blnValid && frmMain.elements[i].valid;            
          }
      }
      
      return blnValid;
  }
  
  //if Ajax is enabled, disable the submit button and assign event handlers
  window.onload = function () {
      if (zXmlHttp.isSupported()) {
          var btnSignUp = document.getElementById("btnSignUp");        
          var txtUsername = document.getElementById("txtUsername");
          var txtBirthday = document.getElementById("txtBirthday");
          var txtEmail = document.getElementById("txtEmail");
  
          btnSignUp.disabled = true;
          txtUsername.onchange = validateField;
          txtBirthday.onchange = validateField;
          txtEmail.onchange = validateField;
          txtUsername.valid = false;
          txtBirthday.valid = false;
          txtEmail.valid = false;
          
      }
  };