One easy JavaScript function that will check or uncheck all the checkboxes in a form that's being passed as the first argument to the function.

1. // To check all: CheckUncheckAll(‘myFormId’, true);
2. // To uncheck all: CheckUncheckAll(‘myFormId’, false);
3. function CheckUncheckAll(formID, checkAll)
4. {
5.  var currForm = document.getElementById(formID);
6.  for (var i = 0; i < currForm.elements.length; i++)
7.  {
8.                 currForm.elements[i].checked = checkAll;
9.  }
10. }