Display a Message or Confirmation Box in C#.net !

HI fellows.,
this is wot my page has....
<Updatepanel>

<datalist>
      <panel>
           <table>
                   <checkboxlist></checkboxlist>
           </table>
      </panel>
</datalist>
</Updatepanel>


The problem now is when i use the following code in the " checkboxlist - SelectedIndexChanged" event.,
Response.Write(@"<script language='javascript'>alert('Hi');</script>");

I get the following error.,
" sys.webforms.pagerequestmanagerservererrorexception ...