Ce l'ho fatta. Metto a disposizione il codice (scusate, non so come metterlo in una box)

<tr >
<td width="150" class="left" valign="top" align="right">
<label>Tipo lastra <span class="required">*</span></label>
</td>
<td class="right">
<select class="other" name="q12_Tipolastra" id="q12" onblur="validate(this,'Required')"

onChange="displaydesc(document.q_form_92814622335. q12_Tipolastra, thetext1,

'textcontainer1')">
<option selected value="L100X140 C">Lastra per stampare schifezze</option>
<option value="lastra2">Lastra2</option>
<option value="L140xxl" target="newwin">Lastra formato 140x140</option>
<option value="lastra4">lastra4</option>
<option value="L100X140 C" target="newwin">L100X140 C</option>
</select>
</td>
</tr>
<tr>
<td style="width:700px;">
<span id="textcontainer1" align="left" style="font:italic 13px Arial">
</span>
</td>
</tr>
<tr >
<td width="150" class="left" align="right">
<label >Quantità <span class="required">*</span></label>
</td>
<td class="right" >
<input type="text" size="20" name="q14_Quantit" class="text" value=""

id="q14"onmouseover="ddrivetip('Numero di lastre ordinate', 200)"

onmouseout="hideddrivetip()" onblur="validate(this,'Required')" onkeyup="HandleKey()"

maxlength="100" maxsize="100" />
</td>
</tr>
<tr >
<td width="150" class="left" align="right">
<label >Numero ordine <span class="required">*</span></label>
</td>
<td class="right" >
<input type="textbox" size="20" name="q15_Numeroordine" class="text" value="" id="q15"

onblur="validate(this,'Required')" onkeyup="HandleKey()" maxlength="100" maxsize="100" />
</td>
</tr>

<tr>
<td width="150" class="left" align="right">
<script language="Javascript">

function HandleKey()
{
document.all.Somma.value=(document.all.q12.value)+ (document.all.q14.value)+(document.all.q15

.value);
}
</script>


<label>Codice a barre</label>
</td>
<td class="right"><input type="textbox" ID="Somma"">
</td>
</tr>