arrivo in ritardo, ma visto che l'ho fatto
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>

    <script language="javascript" type="text/javascript">
// <!CDATA[


function Checkbox1_onclick(v) 
{
    SetAllCheckBoxes("form1", "Checkbox", v.checked);
}

function SetAllCheckBoxes(FormName, FieldName, CheckValue)
{//debugger;
    if(!document.forms[FormName]) return;
    var objCheckBoxes = document.forms[FormName].elements[FieldName];
    if(!objCheckBoxes) return;
    var countCheckBoxes = objCheckBoxes.length;
    if(!countCheckBoxes)
        objCheckBoxes.checked = CheckValue;
    else
        // set the check value for all check boxes
        for(var i = 0; i < countCheckBoxes; i++)
            objCheckBoxes[i].checked = CheckValue;
}



// ]]>
    </script>
</head>
<body>
    <input id="Checkbox1" type="checkbox" onclick="return Checkbox1_onclick(this)" />
    <form id="form1" name="form1" action="?">
        <table border="1">
            <tr><td><input id="Checkbox2" name="Checkbox" type="checkbox" /></td></tr>
            <tr><td><input id="Checkbox3" name="Checkbox" type="checkbox" /></td></tr>
            <tr><td><input id="Checkbox4" name="Checkbox" type="checkbox" /></td></tr>
        </table>
    </form>
</body>
</html>