Expiration problem

Hi everybody, this is my first time writing in this list. I have the next problem: I have a page called login.php and when I log in, there's a new page who receive the user and the password, and this page is called main.php This page, check if the user have a valid account and a password; and if this is OK, then make a session for the user. This work OK; but, from the main.php I go to a link in the same page, but with a new parameter, for example: <a href="main.php?option=1"> and the page showme another screen (because this new screen only appears when I put the "option=1" as parameter); and this also OK. The problem appears when I make click on the BACK BUTTON from my browser, and the browser go back from "main.php?option=1" to the "main.php" page, and get me a error: the page was expire, you have to click REFRESH button on your browser to send the information again. (( or sometime like that; I don't remember the exacts words, 'cause is a spanish Internet Explorer )) Can somebody helpme with this problem? Thanks Gustavo Pereira