i've had some luck with this function...don't remember where i got it and i'm not sure it will help...
function jta_download($file,$filename){
$size=filesize(urldecode($file));
$attachment=(
strpos($HTTP_USER_AGENT,"Mozilla/4")===false // Not Moz4
|| // or
strpos($HTTP_USER_AGENT,"MSIE")!==false) // is IE
? ' attachment;'
: '';
$content_disposition = 'Content-Disposition: attachment; filename=' . $filename;
header($content_disposition);
header("Content-Type: application/octet-stream");
header("Content-Length: $size");
header("Content-Transfer-Encoding: binary");
readfile($file);
}