在后台设置了飘窗广告，但是它是满网页的飘，怎么改成只在屏幕范围内飘？下面是控制飘窗的代码：
<%if(Ad2==1){%>
document.write("<div id='adv_<%=Id%>' style='display:none;position:absolute;left:0px;top:0px;z-index:100'><%=Adv_content%></div>");
var adv_<%=Id%>=document.getElementById("adv_<%=Id%>");
var floatdelay<%=Id%>=1; //控制每次执行间隔的时间，做越大移动得越慢；
var floatspeed =1; //控制每次执行移动的距离，值越大移动得越快；
var floatflagX<%=Id%> =0;
var floatflagY<%=Id%> =0;
function toPixel<%=Id%>(str1)
{
var oldLen = str1.length;
var newLen = oldLen - 2;
str2 = str1.slice(0, newLen);
str3 = parseInt(str2);
return str3;
}
function flowAdv<%=Id%>(){
var bWidth = document.body.clientWidth;
var bHeight = document.body.clientHeight;
var bLeft = document.body.scrollLeft;
var bTop = document.body.scrollTop;
var iWidth = adv_<%=Id%>.offsetWidth;
var iHeight = adv_<%=Id%>.offsetHeight;
var iLeft = toPixel<%=Id%>(adv_<%=Id%>.style.left);
var iTop = toPixel<%=Id%>(adv_<%=Id%>.style.top);
if(iLeft < (bWidth - iWidth) && floatflagX<%=Id%> == 0) {
adv_<%=Id%>.style.left = (iLeft + floatspeed) + "px";
}
else if(iLeft >= (bWidth - iWidth) && floatflagX<%=Id%> ==0) {
floatflagX<%=Id%> = 1;
}
else if(iLeft > 0 && floatflagX<%=Id%> == 1) {
adv_<%=Id%>.style.left = (iLeft - floatspeed) + "px";
}
else if(0 >= iLeft && floatflagX<%=Id%> == 1) {
floatflagX<%=Id%> = 0;
}
if(iTop < (bHeight - iHeight) && floatflagY<%=Id%> == 0) {
adv_<%=Id%>.style.top = (iTop + floatspeed) + "px";
}
else if(iTop >= (bHeight - iHeight) && floatflagY<%=Id%> ==0) {
floatflagY<%=Id%> = 1;
}
else if(iTop > 0 && floatflagY<%=Id%> == 1) {
adv_<%=Id%>.style.top = (iTop - floatspeed) + "px";
}
else if(0 >= iTop && floatflagY<%=Id%> == 1) {
floatflagY<%=Id%> = 0;
}
}
function startflowAdv<%=Id%>()
{
adv_<%=Id%>.style.left =Math.floor(Math.random()*1000)+"px";
adv_<%=Id%>.style.display="block";
var floatinterval<%=Id%> = setInterval("flowAdv<%=Id%>()", floatdelay<%=Id%>);
adv_<%=Id%>.onmouseover = function() {clearInterval(floatinterval<%=Id%>);}
adv_<%=Id%>.onmouseout = function() {floatinterval<%=Id%> = setInterval("flowAdv<%=Id%>()", floatdelay<%=Id%>);}
}
if(Adv_range_<%=Id%>!="0"){setTimeout('startflowAdv<%=Id%>()',Math.floor(Math.random()*1000));}
<%}%>
