Onsubmit cancel

I'm trying to validate form data.  My functions seem to be working, but when returning false, the form action still takes place.

<Script Language = "JavaScript" type = "text/javascript" > 
      function validate_required(field,alerttxt)
      {
            with (field)
            {
                  if (value==null||value=="")
                  {
                        alert(alerttxt);
                        return false;
                  }
                  else
                  {
                        return true;
                  }
            }
      }
            
      function formvalid(thisform)
      {
            with (thisform)
            {
                  if (validate_required(disk_number, "Disk Number must be entered")==false)
                  {
                        id.focus();
                        return false;
                  }
                  else
                  return true;
            }
      }
</script>

<form name="form_input" method="post" onsubmit="return formvalid(this)" action="input.asp">

if formvalid() returns false, shouldn't the action be aborted?  If I leave 'disk_number' blank, I get an alert with the approriate message, but the 'input.asp' still executes.

any ideas?