PHP Pop-Up Javascript

Hi All,

I am looking to create a pop-up message the will broadcast over my site. I want to be able to easily update this message. Below is the code I am currently using to achieve the pop-up

<script type="text/javascript"> 
function popup(URL) { 
day = new Date(); 
id = day.getTime(); 
eval("page" + id + " =, '" + id + "', 'toolbar=0,scrollbars=0,location=0,statusbar=0,menubar=0,resizable=0,width=440,height=420,left = 740,top = 425');"); 

<body onload="javascript:popup('message.jpg')"> 

The problem with the code above is that it is in the plain pop-up window, and if you click on the window behind it the when just get minimized into your task bar.
I would like to have the pop-pop come up as n overlay, and you have to cloe it before you can use the window below. Secondly, this way I can not disable the message with out have to go into each page on which the message needs to be displayed and then enditing the code. Has anyone got any suggestions here?