function cancelBack()
{
	var ctrl  = window.event.ctrlKey;
	var tecla = window.event.keyCode; 
	if (tecla==116) 
	{
		window.event.keyCode=0; 
		window.event.returnValue=false;
	}
	
	if (tecla==117) 
	{
		window.event.keyCode=0; 
		window.event.returnValue=false;
	}
	
	if (tecla==122) 
	{
		window.event.keyCode=0; 
		window.event.returnValue=false;
	}			
	
	if ((event.keyCode == 8 && (event.srcElement.form == null || event.srcElement.isTextEdit == false)) ||
		 event.keyCode == 37 && event.altKey ||
		 event.keyCode == 39 && event.altKey )
	 {
		 event.cancelBubble = true;
		 event.returnValue = false;
	 }			

}