Prova così...:

if(ereg("MSIE ([0-9].[0-9]{1,2})", $_SERVER["HTTP_USER_AGENT"])) {

header("Content-Type: application/octetstream");

header("Content-Disposition: inline; filename=prova.xls");

header("Expires: 0");

header("Cache-Control: must-revalidate, post-check=0, pre-check=0");

header("Pragma: public");

} else {

header("Content-Type: application/octet-stream");

header("Content-Disposition: attachment; filename=prova.xls");

header("Expires: 0");

header("Pragma: no-cache");

}