function SetAllCheckBoxes(FormName, FieldName, CheckValue)
    {
    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;
    }
     
    <form method="GET" action="page17.php" name="myForm" onsubmit="return false;">
    <label for="myCheckbox1">
    <input type="checkbox" name="myCheckbox" value="1" id="myCheckbox1">
    I like Britney Spears </label>
    <br>
    <label for="myCheckbox2"><input type="checkbox" name="myCheckbox" value="2" id="myCheckbox2">
    I like Hillary Duff </label>
    <br>
    <label for="myCheckbox3"><input type="checkbox" name="myCheckbox" value="3" id="myCheckbox3">
    I like Mandy Moore </label>
    <br><input type="button" onclick="SetAllCheckBoxes('myForm', 'myCheckbox', true);" value="I like them all!">
    &nbsp;&nbsp;
    <input type="button" onclick="SetAllCheckBoxes('myForm', 'myCheckbox', false);" value="I don't like any of them!">
    </form>