<form onsubmit='return check()'>
<input type='checkbox' name='cb'/>
<input type='checkbox' name='cb'/>
<input type='checkbox' name='cb'/>
<input type='checkbox' name='cb'/>
<input type='checkbox' name='cb'/>
<input type='checkbox' name='cb'/>
<input type='checkbox' name='cb'/>
<input type='submit'/>
</form>
<script type='text/javascript'>
function check(o)
{
    var msg = "";
    var checkboxes = document.getElementsByName('cb');
    for(var i = 0 ; i < checkboxes.length ; i ++)
    {
        if(checkboxes[i].checked) msg +='第'+(i + 1) + "个被选中\n";
    }
    alert(msg)
}
</script>