function setLinkTargets() {
  var links = document.getElementsByTagName('a');
  for (i=0; i<links.length; i++) {
    if (links[i].href.substring(0,7) == 'http://') {
      links[i].target = '_blank';
    }
  }
}

function addOnloadEvent(func) {
  var oldonload = window.onload;
  if (typeof window.onload != 'function') {
    window.onload = func;
  } else {
    window.onload = function() {
      oldonload();
      func();
    }
  }
}

addOnloadEvent(setLinkTargets);