function event_false() { window.event.returnValue = false }

function derecha(e)
{
 if (navigator.appName == 'Netscape' && (e.which == 3 || e.which == 2))
 {
  alert(msj)
  return false;
 }
 else if (navigator.appName == 'Microsoft Internet Explorer' && (event.button == 2))
 {
  alert(msj)
 }
}

document.onmousedown=derecha
document.onselectstart=event_false