div over entire page

I am trying to put a div over an entire page that already has content on it. Here is the page that I am starting with http://jdprofessionaltraining.com/uvc/photogallery.html and here is a very quick image i put together to give you an idea of what i am looking to be done with the div. I think I already have a solution on where the div will be when not in use. Probably have this <div id="pgholder"></div> and then with javascript put the div that makes that box in there then pt the content in with javascript. I already know how to do the javascript just I am not entirely sure about the css. I am hoping that I can have the div take up the entire window with the exception of a small border that you can see in that image. Does anyone have any ideas?
Thanks

I read over this post twice and still do not understand your problem or what you're asking.

Oops, I forgot to add the image to show you, here it is http://jdprofessionaltraining.com/uvc/photogallery_idea.jpg

I will try to make this clearer. Basically I am trying to put a "window" over content that is already on the page. Like in the jpg i posted I am looking to have a border around the "window", but the border would probably just be a margin for the div. I also would want it to extent to the bottom of the window. I would not even have any idea of where to begin with this?