I would use existing classes such as lightbox, thickbox, and greybox. Less to think about, easy to get going. There are others out there I believe too, but cant remember right off the top of my head.
You could always study the mentioned classes, and then create your own.
As mentioned, would have to use CSS to place the div in the browser window.
As for javascript, its mostly designers and standard web users that have it enabled. Most businesses usually disable it for various reasons. But then dont forget mobile devices, text only browsers, they dont all support javascript.