Code: 

var ie = document.all&&!window.opera?1:0; 
var dom = document.getElementById?1:0; 
var ns4 = document.layers?1:0; 


function init() { 

if(!(ie||dom||ns4)) { 
return; 
} 
else if(ie) { 

box = document.all.dropbox.style; 
boxheight = document.all.dropbox.offsetHeight; 
} 
else if(dom) { 

box = document.getElementById("dropbox").style; 
boxheight = document.getElementById("dropbox").offsetHeight; 
} 
else if(ns4) { 

box = document.dropbox; 
boxheight = box.clip.height; 
} 

scroll_top=(ie)?document.body.scrollTop:window.pageYOffset; 
box.top = scroll_top - boxheight; 
speed = 1; 
dropIn(); 
} 


function dropIn() { 

speed+=6; 

if(parseInt(box.top) < scroll_top ) { 

  box.top = parseInt(box.top) + speed; 
  in_timer = setTimeout("dropIn()", 50); 
} 
else { 
  clearTimeout(in_timer); 
} 
} 


function dropOut() { 

if(parseInt(box.top) > (-1)*boxheight) { 

  box.top = parseInt(box.top) - 40; 
  out_timer = setTimeout("dropOut()", 50); 
} 
else { 
  clearTimeout(out_timer); 
} 
} 

window.onload=init;