function FloatLayer(layerName, y) { 
var t_y = document.documentElement.scrollTop + y; 
eval("document.all." + layerName + ".style.posTop = t_y"); 
} 

var content = "\
<div id=\"FLayer\" style=\"Z-INDEX: 2; right:5px; top:140px; width:100px; height:400px; POSITION: absolute;\">\n \
<a href=\"caisedoufu.asp\" ><img alt=\"彩色豆腐\" src=\"img/color_doufu.jpg\" width=\"100\" /></a>\n \
</div>";

document.write(content);
setInterval("FloatLayer('FLayer',140);", 20);