Salve

il mio probblema è il seguente, ho una form come questa:

codice:
<form method="post" name="form1" action="<?php echo $editFormAction; ?>">

        <table width="737" align="center">
          <tr valign="baseline"> 
            <td width="110" align="right" nowrap>Id:</td>
            <td colspan="6"><input type="text" name="id" value="" size="5"> <font size="1" face="Comic Sans MS">&lt;-Non 
              Modificare</font></td>
          </tr>
          <tr valign="baseline"> 
            <td nowrap align="right">Fornitore:</td>
            <td width="193"><select name="fornitore" id="fornitore">
                <?php
  while(!$Recordset1->EOF){
?>
                <option value="<?php echo $Recordset1->Fields('Fornitore')?>"<?php if (!(strcmp($Recordset1->Fields('Fornitore'), $Recordset5->Fields('fornitore')))) {echo "SELECTED";} ?>><?php echo $Recordset1->Fields('Fornitore')?></option>
                <?php
    $Recordset1->MoveNext();
  }
  $Recordset1->MoveFirst();
?>
              </select> [img]../immagini/doc.gif[/img] 
            </td>
            <td colspan="2">Data Registrazione:</td>
            <td colspan="3"> <input type="text" name="data_registrazione" id="cal_fld_data_registrazione[data_registrazione]"value="<?php echo date('d-m-Y');?>" size="9"> 
              [img]calendar.png[/img] <script language="JavaScript">
 <!--
 Calendar.setup({
 range: [2003,2050], 	inputField: "cal_fld_data_registrazione[data_registrazione]",
 	button: "cal_btn_data_registrazione[data_registrazione]",
 	ifFormat: "dd-mm-y",
 	daFormat: "dd-mm-y",
 	singleClick: false,
 	align: "Tl",
 	weekNumbers: false,
 	mondayFirst: true
 });
 //-->
 </script></td>
          </tr>
          <tr valign="baseline"> 
            <td nowrap align="right">Tipo Documento:</td>
            <td> <select name="tipo_doc" id="select">
                <option value="Fattura">Fattura</option>
                <option value="Nota Credito">Nota Credito</option>
              </select></td>
            <td colspan="2">Numero Doc.:</td>
            <td width="58"><input type="text" name="n_fattura" value="" size="6"></td>
            <td width="36">Nave:</td>
            <td width="179"> <select name="nave" id="select2">
                <?php
  while(!$navi->EOF){
?>
                <option value="<?php echo $navi->Fields('NAVE')?>" selected><?php echo $navi->Fields('NAVE')?></option>
                <?php
    $navi->MoveNext();
  }
  $navi->MoveFirst();
?>
              </select></td>
          </tr>
          <tr valign="baseline"> 
            <td nowrap align="right">Data Documento :</td>
            <td><input type="text" name="data" id="cal_fld_data_a[data_a]" value="" size="9"> 
              [img]calendar.png[/img] <script language="JavaScript">
 <!--
 Calendar.setup({
 range: [2003,2050], 	inputField: "cal_fld_data_a[data_a]",
 	button: "cal_btn_data_a[data_a]",
 	ifFormat: "dd-mm-y",
 	daFormat: "dd-mm-y",
 	singleClick: false,
 	align: "Tl",
 	weekNumbers: false,
 	mondayFirst: true
 });
 //-->
 </script></td>
            <td width="61">Dare:</td>
            <td width="68"><input type="text" name="dare" value="" size="6"></td>
            <td>Avere:</td>
            <td><input type="text" name="avere" value="" size="6"></td>
            <td></td>
          </tr>
          <tr valign="baseline"> 
            <td nowrap align="right"></td>
            <td colspan="6"><?php echo $data;?></td>
          </tr>
          <tr valign="baseline"> 
            <td nowrap align="right"></td>
            <td colspan="6"><div align="center"> 
                <input name="submit" type="submit" value="Inserisci record">
                <input type="reset" name="Submit" value="Reimposta">
              </div></td>
          </tr>
        </table>

        <input type="hidden" name="MM_insert" value="form1">

      </form>
ora io vorrei che il campo dare sia disattivo e che venga attivato quando dal campo select "tipo_doc" viene selzionato "nota credito".
mi potete aiutare grarie 1000