AJAX Loading Picture

I need a fully integrateable script for my Magento Webshop (ver. 1.3.2.4) that does the following: Evertime a page loads a semi-transparent layer (dark grey, something like that) shall cover the whole page, in the centre an animated gif shall appear and further clicks shall not be possible for the user until the page has fully loaded. I already have the animated gif picture, I just need the code and the position to integrate it into Magento.