a_div = document.getElementById ? document.getElementById('if_actual_div') : document.all.if_actual_div;

             var screen_width = document.documentElement.clientWidth;
             var fl_pos = ((screen_width - 800) / 2) + 355;

             if (screen_width <= 800) {
                a_div.style.left = 355;
             } else {
                a_div.style.left = fl_pos;
             }