#upBlock {
position:fixed;
top:15px;
left:15px;
background: #FFDDBB;
color:#000000;
padding:6px;
width: 45px;
text-align:center;
cursor:pointer;
filter:progid:DXImageTransform.Microsoft.Alpha(opacity=5);
-moz-opacity: 0.05;
-khtml-opacity: 0.05;
opacity: 0.05;
}

<script>
$(document).ready(function(){
var opti = 0;
$(window).scroll(function(){
var scro = $(this).scrollTop();

if(scro > 100 && opti == 0){ $("#upBlock").animate({opacity:0.5},600); opti = 0.5; }
if(scro > 300 && opti != 1){ $("#upBlock").animate({opacity:1},600); opti = 1; }

if(scro < 100 && opti > 0){ $("#upBlock").animate({opacity:0.05},600); opti = 0; }
if(scro < 300 && opti == 1){ $("#upBlock").animate({opacity:0.5},600); opti = 0.5; }

});
$("#upBlock").click(function () {
$(window).scrollTop(0);
$("#upBlock").animate({opacity:0.05},600); opti = 0;
});

});
</script><div id="upBlock">Наверх</div>