AJAX File Download

A file download initialized by a button click on a button that is placed inside an updatepanel requires some fiddling with IFRAME

The reason for this is:

"Since there is no traditional HTTP response, you have no context with which to send the file to the browser for normal download."

Check this out for a complete solution: http://encosia.com/2007/02/23/ajax-file-downloads-and-iframes/