IE默认的font-size：12px会影响盒子的高度,除非你盒子里面有文字，
以后记得在没有内容的DIV里面加上 font-size:2px
所以你只要给 .main 加上 font-size:0px就OK了。
当然，.main里面要加内容的话，你可以把 font-size:0px放到各个定角的DIV。
以下是我调整过的：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/htmlcharset=utf-8" />
<title>无标题文档</title>
<link rel="stylesheet" type="text/css" href="ChatWindow.css" />
<script type="text/javascript" language="javascript">
</script>
</head>
<style>
*
{
margin:0px
padding:0px
border:0px
}
.main
{
position:absolute
top:20px
left:20px
border:1px solid black
width:450px
height:500px
font-size:0px
}
.top_border
{
position:absolute
left:3px
top:0px
width:444px
height:3px
background-color:#FFCC00
cursor:n-resize
}
.left_border
{
position:absolute
left:0px
top:3px
height:494px
width:3px
background-color:red
cursor:e-resize
}
.right_border
{
position:absolute
right:0px
top:3px
width:3px
height:494px
background-color:#009900
cursor:e-resize
}
.bottom_border
{
position:absolute
bottom:0px
left:3px
width:444px
height:3px
background-color:#660000
cursor:n-resize
}
.right_bottom
{
position:absolute
right:0
bottom:0
width:3px
height:3px
background-color:#000000
cursor:nw-resize
}
.left_bottom
{
position:absolute
left:0
bottom:0
width:3px
height:3px
background-color:#000000
cursor:ne-resize
}
.left_top
{
position:absolute
left:0
top:0
width:3px
height:3px
background-color:#000000
cursor:nw-resize
}
.right_top
{
position:absolute
right:0
top:0
width:3px
height:3px
background-color:#000000
cursor:ne-resize
}
</style>
<body>
<div class="main">
<div id="top_border" class="top_border"></div>
<div class="left_border"></div>
<div class="right_border"></div>
<div class="bottom_border"></div>
<div class="right_bottom"></div>
<div class="left_bottom"></div>
<div class="left_top"></div>
<div class="right_top"></div>
<div class="title"></div>
<div class="chat_note"></div>
<div class="tools"></div>
<div class="editer"></div>
<div class="footer"></div>
</div>
</body>
</html>
