var l = (screen.width-480)/2; 
var t = (screen.height-430)/2;

function comenta(postid) {	
  window.open("http://www.samila.com.br/kelly/bitch/comentarios.php?postid="+postid, "", "height=430,width=470,left="+l+",top="+t+",scrollbars=1");
}