prova così

codice:
function anteprimaa(theURL,winName,features){

  // if (document.all || document.layers)
  w = screen.availWidth;
  h = screen.availHeight;

  var popW = 400;
  var popH = 200;
  var leftPos = parseInt(((w-popW)/2), 10);
  var topPos = parseInt(((h-popH)/2), 10);

  window.open('anteprime/prima.htm','newWindow','width=' + popW + ',height=' + popH + ',top=' + topPos + ',left=' + leftPos,'toolbar=0,location=0,status=0,menubar=0,scrollbars=0,resizable=0');

}
Ciao