Assumes jQuery. This will detect the position of your ID, check if it's got a margin on the top and remove it. If the ID reaches the top of the screen, then it will add a class fixed. Obviously you would need to make position:fixed; part of the fixed class, plus add in the actual fixed position.

affix-to-top.js

$(function () {  
  var top = $('#ID').offset().top - parseFloat($('#ID').css('margin-top').replace(/auto/, 0)); // math is there in case the element has margin-top and removes it
  $(window).scroll(function (event) {
    // what the y position of the scroll is
    var y = $(this).scrollTop();    
    // whether that's below the form
    if (y >= top) {
      // if so, ad the fixed class
      $('#menu').addClass('fixed');
    } else {
      // otherwise remove it
      $('#menu').removeClass('fixed');
    }
  });
});