function addOnloadEvent(){

  if ( typeof window.addEventListener != "undefined" ) {
    window.addEventListener( "load", externalLinks, false );
  }
  else if ( typeof window.attachEvent != "undefined" ) {
    window.attachEvent( "onload", externalLinks );
  }
  else {
    if ( window.onload != null ) {
      var oldOnload = window.onload;
      window.onload = function ( e ) {
        oldOnload( e );
        window[externalLinks]();
      };
    }
    else {
      window.onload = externalLinks;
	}
  }
}

function externalLinks() {
 if (!document.getElementsByTagName) return;   
 var anchors = document.getElementsByTagName("a");   
 for (var i=0; i<anchors.length; i++) {
   var anchor = anchors[i];
   if (anchor.getAttribute("href") &&   
       anchor.getAttribute("rel") == "external")   
     anchor.target = "_blank";   
 }   
}   
//window.onload = externalLinks;
addOnloadEvent();