I've found other solution on how to executes a javascript function from code-behind. The problem is that it firing before the page loads, so I can't add logic to it.
if(result)
{
   Page.ClientScript.RegisterClientScriptBlock(typeof(Page), 
                                               "Script", "confirmPwdChange();", true);
}
On the client side
<script>
   confirmPwdChange()
   {
     alert("Password has been successfully changed.");
     //Add more logic here, after user clicks the OK button...
   }
</script>
No Jquery, if possible.
Thanks for helping.
 
     
    