Non l'ho testato pienamente...

Codice PHP:
<html>
    <head>
        <title>Home</title>
        <script type="text/javascript">
            function calc()
            {
                var campo;
                var divisori;
                var numero;
                campo=document.getElementById("val");
                divisori=document.getElementById("div");
                numero=document.getElementById("num");
                val=campo.value;
                numero.value="";
                divisori.value="";
                if(val>0)
                {
                    numero.value=val;
                    for (i=1; i<=val; i++)
                    {
                        if (val%i==0)
                        {
                            divisori.value=divisori.value+" "+i;
                        }
                    }
                }
                else
                {
                    alert("Valore non valido");
                    numero.value="Dati non validi";
                    divisori.value="Dati non validi";
                }
            }
        </script>
    </head>
    <body>
        <form name="frm">
            <input type="text" name="val" id="val">
            <input type="button" name="run" value="Ok" onClick="return calc()">
            <table width="200" border="1">
                <tr>
                    <td>[b]Valore:[/b]</td>
                    <td><textarea readonly="readonly" name="num" id="num"></textarea></td>
                </tr>
                <tr>
                    <td>[b]Divisori:[/b]</td>
                    <td><textarea readonly="readonly" name="div" id="div"></textarea></td>
                </tr>
            </table>
        </form>
    </body>
</html>