Most simple way is to use MSAjax pageLoad Event in your javascript code :
<script>
///<summary>
/// This will fire on initial page load,
/// and all subsequent partial page updates made
/// by any update panel on the page
///</summary>
function pageLoad(){ alert('page loaded!') }
</script>
I have used it many times, it works like charm. Most important thing is don’t confuse it with document.ready function (which will be executed only once after the page Document Object Model (DOM) is ready for JavaScript code to execute),yet pageLoad Event will get executed every time the update panel refreshes.