how do they make flash animated windows layered over a web page??

I've seen these flash windows on websites that are layered over a web page. When you scroll down they scroll down along with you or have other neat features. Yesterday I saw one that was rotated and its edges were transparant so I could see the webpage below it.

Does ANYONE know how they do this? I'd like to find a tutorial on this and create a cool layoring effect for my webiste.

Thanks!!!

Hi
I think when you go to Publish Settings there is an option to have 'windowless' and this makes your swf transparent, leaving the html page visible underneath...
But search the forums for more information, as I'm sure others have gotten a fuller answer to the same question in the past.
Regards