function preventSelection(element){
  var preventSelection = false;

  function addHandler(element, event, handler){
  if (element.attachEvent) 
  element.attachEvent('on' + event, handler);
  else 
  if (element.addEventListener) 
  element.addEventListener(event, handler, false);
  }
  function removeSelection(){
  if (window.getSelection) { window.getSelection().removeAllRanges(); }
  else if (document.selection && document.selection.clear)
  document.selection.clear();
  }
  function killCtrlA(event){
  var event = event || window.event;
  var sender = event.target || event.srcElement;

  if (sender.tagName.match(/INPUT|TEXTAREA/i))
  return;

  var key = event.keyCode || event.which;
  if (event.ctrlKey && key == 'A'.charCodeAt(0)) // 'A'.charCodeAt(0) можно заменить на 65
  {
  removeSelection();

  if (event.preventDefault) 
  event.preventDefault();
  else
  event.returnValue = false;
  }
  }
  addHandler(element, 'mousemove', function(){
  if(preventSelection)
  removeSelection();
  });
  addHandler(element, 'mousedown', function(event){
  var event = event || window.event;
  var sender = event.target || event.srcElement;
  preventSelection = !sender.tagName.match(/INPUT|TEXTAREA/i);
  });
  addHandler(element, 'mouseup', function(){
  if (preventSelection)
  removeSelection();
  preventSelection = false;
  });
  addHandler(element, 'keydown', killCtrlA);
  addHandler(element, 'keyup', killCtrlA);
}

preventSelection(document);