Código HTML:
<!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" xml:lang="en" lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=iso-8859-1" />
<meta name="author" content="Jackson666" />
<title>Untitled 2</title>
<style type="text/css">
*{margin: 0;padding: 0;}
#barra{background-color: black; height:20px; position: absolute; left: 0; top: 20px;}
#contenedor{position: relative; margin: auto; height: 60px; width: 100%;}
</style>
</head>
<body>
<div id="contenedor">
<div id="barra"></div>
</div>
<script type="text/javascript">
var x=0;
window.onload=progress();
function progress(){
var timer;
if(x<100){
x++;
document.getElementById('barra').style.width=x+'%';
timer=setTimeout('progress()',50);
}else{
clearTimeout(timer);
location.href='tuPagina.htm';
}
}
</script>
</body>
</html>