ho capito dove sbagliavo, ho fatto un gran casino.
Ora mi funziona tutto:

<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<select name="elenco"
onchange="window.open(this.options.value,'_top')" size="1">
<option selected value="seleziona">seleziona</option>
<option value="alfaromeo.htm">ALFA ROMEO</option>
<option value="audi.htm">AUDI</option>
<option value="bmw.htm">BMW</option>
<option value="chevrolet.htm">CHEVROLET</option>
<option value="citroen.htm">CITROEN</option>
<option value="daewoo.htm">DAEWOO</option>
<option value="ferrari.htm">FERRARI</option>
<option value="fiat.htm">FIAT</option>
<option value="ford.htm">FORD</option>
<option value="honda.htm">HONDA</option>
<option value="hyundai.htm">HYUNDAI</option>
<option value="jaguar.htm">JAGUAR</option>
<option value="kia.htm">KIA</option>
<option value="lada.htm">LADA</option>
<option value="lamborghini.htm">LAMBORGHINI</option>
<option value="lancia.htm">LANCIA</option>
<option value="lexus.htm">LEXUS</option>
<option value="lotus.htm">LOTUS</option>
<option value="maserati.htm">MASERATI</option>
<option value="mazda.htm">MAZDA</option>
<option value="mercedes.htm">MERCEDES</option>
<option value="mitsubishi.htm">MITSUBISHI</option>
<option value="nissan.htm">NISSAN</option>
<option value="opel.htm">OPEL</option>
<option value="peugeot.htm">PEUGEOT</option>
<option value="porsche.htm">PORSCHE</option>
<option value="renault.htm">RENAULT</option>
<option value="rover.htm">ROVER</option>
<option value="saab.htm">SAAB</option>
<option value="seat.htm">SEAT</option>
<option value="skoda.htm">SKODA</option>
<option value="smart.htm">SMART</option>
<option value="subaru.htm">SUBARU</option>
<option value="suzuki.htm">SUZUKI</option>
<option value="toyota.htm">TOYOTA</option>
<option value="volkswagen.htm">VOLKSWAGEN</option>
<option value="volvo.htm">VOLVO</option>
</select>

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