var x = 50,y = 60
var xin = true, yin = true
var step = 1 
var delay = 10
var obj=document.getElementById("ad") 
function floatAD() {
var L=T=0
var R= document.body.clientWidth-obj.offsetWidth
var B = 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)}