handle error msg due back button in IE in only one page of my application

Question

hi
 
I have a problem with the back  button in IE
In my C# application where a page contains dropdownlist and a grid. The  appln is like dropdownlist contains options like  YES,No,Not declared.
when I choose any of the option in dropdownlist the corresponding details sholud be displayed in the grid.
If i use the dropdownlist list options many times any give the back button in IE it comes as page has expired.
 
Pls give me soution where it should either redirect to the previous page or should not redirect to the previous page.
I even tried to disable back btn in IE using the following scripts it works fine on all pages of my application except in this particular page. 
<script type = "text/javascript" >

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

setTimeout("preventBack()", 0);

window.onunload=function(){null};

</script>

 
Pls gie me the solution for this issue