In this article, you learn to create a modal popup login page. AJAX makes it easy to implement. With UpdatePanel, the page won’t post back if login failed. By adding your own authentication logic, you can expand this demo easily to real world application.

Reference

Polished Popups

http://www.aspnetpro.com/articles/2009/05/asp200905so_f/asp200905so_f.asp

This article only shows you how to create a modal popup confirm dialog, not a login dialog. And the panel doesn’t set Style="display: none"; the hidden panel is visible when first time loaded.