I have PHP 4.3 running on a Windows 2003 Server running IIS 6.0 also using SSL encryption for my pages.  I could not (for the life of me) figure out how to get IE 6.0/WinXPPro to recognize a set of HTML tables as an Excel spreadsheet export, and it was due to the header() variables I was using.  Hopefully these are helpful to others who are attempting the same type of export within PHP.

*This example builds on the previously submitted one, adding a few necessary headers.

<?php
    header("Expires: Sat, 01 Jan 2000 00:00:00 GMT");
    header("Last-Modified: ".gmdate("D, d M Y H:i:s")." GMT");
    header("Pragma: public");
    header("Expires: 0");
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
    header("Cache-Control: public"); 
    header("Content-Description: File Transfer");
    
    session_cache_limiter("must-revalidate");
    header("Content-Type: application/vnd.ms-excel");
    header('Content-Disposition: attachment; filename="fileToExport.xls"');

    // and after you start the session
    session_start();
?>