ie6,ie7,ie8完美支持position:fixed的终极解决方案

<div id="goTop_div" class="fixed ie">  </div>

.fixed{
        position:fixed; /*对于火狐等其他浏览器需要设置的*/
        top:700px;  /*同上*/
        width:30px;
        height:30px;
        cursor:pointer;
        display:none;
 } 
 .ie{  
       _position: absolute; 
       _clear: both;  
       _top:expression(eval(document.compatMode &&  
               document.compatMode=='CSS1Compat') ?  
               documentElement.scrollTop  
               +(documentElement.clientHeight-this.clientHeight) - 1  
               : document.body.scrollTop  
               +(document.body.clientHeight-this.clientHeight) - 1);
}