Other people get 404 when trying to access PHP pages of my server. :(

All seems to be working fine locally, using numeric IP or just "localhost" as URL, with Windows XP, Apache and PHP installed (plus mySQL). I can view any pages on my server at home, be it html, php or even a forum based on mySQL. Also I don't have a firewall installed. When I ask other people if they can access my server, they tell me the Apache Welcome message works fine (in general). But any PHP pages either don't display at all or just "flash by" for a split second, to be replaced by a "page cannot be displayed" error (probably a 404). From earlier experiments I know it does work ok with Windows ME, but it doesn't work on Windows XP. I've also found a similar description of this error here: http://bugs.php.net/bug.php?id=14463 So does anyone know how to get PHP to work? Thanks in advance.