fai così:
Codice PHP:<?php
...bla bla
if(mysql_num_rows($query) == 1){
...bla bla
}else{
$error = 'Nome Utente o password errati';
}
...bla bla
?>
<!DOCTYPE html>
<html lang="it">
<head><title></title><head><body>
... form html per il login...
<!-- dove ti pare -->
<?php if(isset($error)):?>
<div style="position:absolute;width:80%;left:50%;margin-left:-40%; top:500px;text-align:center; z-index:999999">
<b><?php echo $error ?></b>
</div><?php endif ?>
</body></html>

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