// this script will disable the user using the browser back button in
/* 1.     Internet Explorer 5.55

2.     Internet Explorer 6

3.     Internet Explorer 7

4.     Mozilla Firefox 3

5.     Opera 9

6.     Safari 4

7.     Google Chrome

og web site: http://www.aspsnippets.com/post/2009/03/24/Disable-Browser-Back-Button-Functionality-using-JavaScript.aspx
*/ 

function preventBack(){window.history.forward();}

    setTimeout("preventBack()", 0);

    window.onunload=function(){null};