you can put anything you want to popin in a div with theese properties:
left:50px; /* you should change left and top */
you should put #popup div right after <body> tag (or just before </body> )
in header tag, add this:
ok, now you have a popIN window Ofcourse, we have to hide the window, right?
for that, you should have a button on your page (let's assume we have <a> tag). You have to do this way:
<a href="#" onclick="document.getElementById('popup').style.display='none'; return false;">CLOSE</a>
And there you go
For animation effect you can use either a library (there is A LOT others) like jquery, or, like mister Diaz tell before, with YUI.
For jquery, you have to change your css to something like top:-500px. The code for that is:
(ofcourse, you MUST include jquery.js
I hope this will help you. And DON'T buy that software