&^%$! Download "accelerators"

I've been using the following code for some time to deliver electronic downloads of purchased software products - header('Content-Type: '.$file_row['content_type']); header('Content-Disposition: filename="'.$file_row["filename"].'"'); $size = filesize('../../store/files/'.$file_row['filename']); header('Content-Length: '.$size); header('Content-Transfer-Encoding: base64'); readfile('../../store/files/'.$file_row['filename']); But once in a blue moon, someone complains that they didn't get their file and they always seem to be using some "download accelerator" software. Does anyone have any experience with these? Anything I can do to make the above code more robust? Note that it's been working great for a long time. - Brian