It's because IE (and other browsers) know that the page you are trying to view was the results of submitting a form (your login page) and they are telling you that they don't want to submit the same form again.
To get by this, validate the login, then use <b>header("Location: XXXXX")</b> to send the user to the next page. This way, when the use the back button, the browser will remember that they were redirected to another page and will not worry about the form.
-- Rich