function initAd() { 
document.all.AdLayer.style.posTop = 0;
document.all.AdLayer.style.posLeft = 0;
document.all.AdLayer.style.visibility = 'visible' 
MoveLayer('AdLayer',1,1); 
} 

function MoveLayer(layerName,signx,signy) { 
var x = 10; 
var y = 10; 

var diffx=2;
var diffy=2;

var adLeft=parseInt(AdLayer.style.left);  
var adTop=parseInt(AdLayer.style.top);  
var adWidth=parseInt(AdLayer.style.width);  
var adHeight=parseInt(AdLayer.style.height)  
var _bodyLeft=document.body.scrollLeft;  
var _bodyTop=document.body.scrollTop;  
var _bodyHeight=document.body.clientHeight+_bodyTop;  
var _bodyWidth=document.body.clientWidth+_bodyLeft;  

if(adLeft<=_bodyLeft){
	if ((adLeft<=_bodyLeft)||(signx==1))
	{
		signx=1;
	}
	diffx=2;  
}  
if((adTop<=_bodyTop)||(signy==1)){  
	if (adTop<=_bodyTop)
	{
		signy=1;
	}
	diffy=2;  
}  
if(((adLeft+adWidth)>=_bodyWidth)||(signx==-1)){  
	if ((adLeft+adWidth)>=_bodyWidth)
	{
		signx=-1;
	}
	diffx=-2  
}  
if(((adTop+adHeight)>=_bodyHeight)||(signy==-1)){  
	if ((adTop+adHeight)>=_bodyHeight)
	{
		signy=-1;
	}
	diffy=-2  
}  



x =adLeft+diffx;
y =adTop+diffy;

eval("document.all." + layerName + ".style.posLeft = x");
eval("document.all." + layerName + ".style.posTop = y"); 
 
setTimeout("MoveLayer('AdLayer',"+signx+","+signy+");", 20);//设置20毫秒后再调用函数MoveLayer() 
}