Creating Modal block, overlay effects with jQuery

I want a Login Box to be opened in modal window when user clicks on a link and authentication is done with Ajax.
I tried jQuery BlockUI, jQuery UI, ThickBox. But they are large in scope.
I also tried writing separate plugin for my need but i don't have clear idea how does it work.

So please either suggest a way or give me link to article which is simple to understand and can clear how it work.

This is about JavaScript. JavaScript is not Java. Try the Scripting - Client Side forum.