function validateForm(f) {
  var first_field = null;
  var result = true;
  var checkbox_checked = false;
  var ncheckbox = 0; //counter of required checkboxes in a group

  try {
    var all = f.elements;
    $A(all).each(function(ifield){

      if (Element.hasClassName(ifield, "required_field_checkbox")) {
        ncheckbox++;
        if(ifield.checked) {
          checkbox_checked = true;
        }
      }

      if (Element.hasClassName(ifield, "required_field")) {
        if('' == ifield.value) { // need more strict empty value verification
          first_field = ifield;
          result = false;
          Element.addClassName(ifield, 'required_field_highlighted');
        } else { // reset to default style of border?
          Element.removeClassName(ifield, 'required_field_highlighted');
        }
      }

    });
    if (first_field) first_field.focus();
  } catch (e) {}

  if (ncheckbox > 0 && !checkbox_checked) {
    result = false;
    alert("Please choose setup a personalized demo or request a free system for 10 users or both");
  }

  return result;
}