mi correggo:


codice:
<html><head>

<script language="JavaScript" type="text/javascript">
 function conta(val) {
	max=100
        if (val.testo.value.length > max) {
	        val.testo.value = val.testo.value.substring(0,max)
		rest = 0
        } 
	else {
		rest = max - val.testo.value.length
		}
        val.num.value = rest
 }
//--> 
</SCRIPT>


</head>
<body>


<form name=modulo   >
<input value="100" size="3" name="num" disabled>
<textarea name=testo onKeyUp="conta(this.form)"></textarea>
</form>

</body>
ecco così funziona..