var x = y = 0;
var xin = true, yin = true
var step  = 1;
var delay = 1;
var obj = document.getElementById("ad"); 
function floatAD(){
 var L = 0;
 var T = (document.body.scrollTop?document.body.scrollTop:document.documentElement.scrollTop);
 var R = document.body.clientWidth-obj.offsetWidth;
 var B = T + ((document.body.clientHeight > document.documentElement.clientHeight)?document.documentElement.clientHeight:document.body.clientHeight) - obj.offsetHeight;
 obj.style.left = x + document.body.scrollLeft;
 obj.style.top = y + document.body.scrollTop;
 x = x + step*(xin?1:-1);
 if (x < L) { xin = true; x = L; } 
 if (x > R){ xin = false; x = R; } 
 y = y + step * (yin?1:-1);
 if (y < T) { yin = true;  y = T; } 
 if (y > B) { yin = false; y = B; } 
}
var itl = setInterval("floatAD()", delay);
obj.onmouseover = function(){ clearInterval(itl) } 
obj.onmouseout  = function(){ itl = setInterval("floatAD()", delay) }