在本机上测试没有问题噢 500px也好 1024px也好 都没发现问题
稍微改了一下css  因为上面的css有些错误(另外觉得速度慢了  把速度改了)
<div id="demo" style="overflow:hidden;height:100%;width:500px;background:#214984;color:#ffffff"> 
<table align=left cellpadding=0 cellspace=0 border=0> 
<tr> 
<td id=demo1 valign=top> <img src="http://img1.qq.com/news/pics/14772/14772522.jpg"><img src="http://img1.qq.com/news/pics/14758/14758448.jpg"><img src="http://img1.qq.com/news/pics/14752/14752986.jpg"><img height="210px" width="310" src="http://img1.qq.com/news/pics/14768/14768491.jpg"> 
</td> 
<td id=demo2 valign=top> </td> 
</tr> 
</table> 
</div> 
<script> 
var speed=2
demo2.innerHTML=demo1.innerHTML 
function Marquee(){ 
if(demo2.offsetWidth-demo.scrollLeft <=0) 
demo.scrollLeft-=demo1.offsetWidth 
else{ 
demo.scrollLeft +=10 


var MyMar=setInterval(Marquee,speed) 
demo.onmouseover=function() {clearInterval(MyMar)} 
demo.onmouseout=function() {MyMar=setInterval(Marquee,speed)} 
</script>