Probleme doctype - bloque script

Bonjour,

j ai deux script different sur ma page, quand je met doctype ca bloque l un des deux et quand je l enleve ou remplace c est l autre qui ne fonctionne plus... helpppp

<!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<head>

<head>
<title>Menu HTML / Javascript : Aperçu</title>
<!-- nettuts -->
<!-- traduit et adapté par outils-web.com -->
<!-- chargement des feuilles de style -->
<link href="css/style.css" rel="stylesheet" type="text/css" />

<script language="JavaScript1.2" type="text/javascript">
if (screen.width==800||screen.height==600)
document.write('<body background="a.jpg">');
else if (screen.width==1024||screen.height==768)
document.write('<body background="b.jpg">');
else if (screen.width==1280||screen.height==1024)
document.write('<body background="c.jpg">');
else if (screen.width==1600||screen.height==1200)
document.write('<body background="d.jpg">');
else
document.write('<body background="d.jpg">');
</script>

</head>

<ul id="nav">
<li class="current"><a href="#">Accueil</a></li><!-- n1 -->
<li><a href="#">Projets</a><!-- n1 -->
<ul>
<li><a href="#">Scripts <img src="images/multi.png" border="0" width="15" height="15" align="absmiddle"></a><!-- n2 -->
<ul>
<li><a href="#">Javascript</a></li><!-- n3 -->
<li><a href="#">jQuery</a></li><!-- n3 -->
</ul>
</li>
<li><a href="#">Diaporamas <img src="images/multi.png" border="0" width="15" height="15" alt="" align="absmiddle"></a><!-- n2 -->
<ul>
<li><a href="#">jQuery</a></li><!-- n3 -->
<li><a href="#">Flash</a></li><!-- n3 -->
</ul>
</li>
<li><a href="#">Lightbox</a></li><!-- n2 -->
<li><a href="#">Kits graphiques</a></li><!-- n2 -->
</ul>
</li>
<li><a href="#">Multi-Niveaux</a>
<ul>
<li><a href="#">niveau 1 <img src="images/multi.png" border="0" width="15" height="15" alt="" align="absmiddle"></a>
<ul>
<li><a href="#">Niveau 2</a></li>
<li><a href="#">Niveau 2 <img src="images/multi.png" border="0" width="15" height="15" alt="" align="absmiddle"></a>
<ul>
<li><a href="#">Niveau 3</a></li>
<li><a href="#">Niveau 3</a></li>
<li><a href="#">Niveau 3</a></li>
</ul>
</li>
<li><a href="#">Niveau 2</a></li>
</ul>
</li>
<li><a href="#">Niveau 1</a></li>
<li><a href="#">Niveau 1</a></li>
<li><a href="#">Niveau 1 <img src="images/multi.png" border="0" width="15" height="15" alt="" align="absmiddle"></a>
<ul>
<li><a href="#">Niveau 2</a></li>
<li><a href="#">Niveau 2</a></li>
<li><a href="#">Niveau 2</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#">Liens</a></li>
<li><a href="#">Contact</a></li>
</ul>
<br /><br /><br /><br /><br /> <p> </p>

<p align="center"><br />
<!--#include file=../footer_scr.asp-->

</head>

<body>
<img name="imgalea" WIDTH="50%" HEIGHT="50%" id="imgalea" "40%" src="img5.jpg" align="center"
border="0" position="fixed" style="filter:progid:DXImageTransform.Microsoft.Fade(Duration=2.0,overlap=1.0)" >
<script language="JavaScript1.2">
var prefix='img'
var extension='jpg'
var nbimages=5
var delai=2800

//==========================

var newi=0

function changeimage()

{

newi = newi+1

if (newi == nbimages+1) newi = 1

if(document.getElementById) o=document.getElementById('imgalea');

else if(document.images) o=document.images.imgalea;

if(o)

{

if(o.filters) o.filters[0].apply();

o.src=prefix+newi+'.'+extension

if(o.filters) document.getElementById('imgalea').filters[0].play();

}

setTimeout("changeimage()",delai)

}

changeimage()

</script>

</body>

</html>

merciiiii