div{
  position:fixed;
  margin:auto;
  left:0;
  right:0;
  top:0;
  bottom:0;
  width:200px;
  height:150px;
}

<style type="text/css">
.dialog{
 position: fixed;
 _position:absolute;
 z-index:1;
 top: 50%;
 left: 50%;
 margin: -141px 0 0 -201px;
 width: 400px;
 height:280px;
 border:1px solid #CCC;
 line-height: 280px;
 text-align:center;
 font-size: 14px;
 background-color:#F4F4F4;
 overflow:hidden;
} 
</style>
<div class="dialog">我是在窗口正中央的，呵呵！</div>

.dialog{
 position: fixed;
 _position:absolute; /* hack for IE6 */
 z-index:1;
 top: 50%;
 left: 50%;
 margin: -141px 0 0 -201px;
 width: 400px;
 height:280px;
 border:1px solid #CCC;
 line-height: 280px;
 text-align:center;
 font-size: 14px;
 background-color:#F4F4F4;
 overflow:hidden;
}