Hi Kim,
usually Moodle replies with HTTP 404 in case of missing required parameters or of sanity check errors (see lib/deprecatedlib.php::error() and lib/weblib.php::print_error(), i.e. reading course/view.php will drive you to each possible error case).
Please, in IE uncheck "Show friendly HTTP error messages": this will eventually show any message from Moodle about the reason for the error (IE show its friendly msgs when the server replies with less than 512 bytes). Something similar for FF: about:config => browser.xul.error_pages.enabled.
Knowing what Moodle claims has an error will help you in finding what these 2 users are different in (permissions, roles, ...).
HTH,
Matteo
