function skip () { 
 this.blur(); 
}

function disableTextField (field) {
  if (document.all || document.getElementById) 
    field.disabled = true;
  else {
    field.oldOnFocus = field.onfocus;
    field.onfocus = skip;
  }
}

function enableTextField (field) {
  if (document.all || document.getElementById)
    field.disabled = false;
  else {
    field.onfocus = field.oldOnFocus;
  }
}