Trying AJAX autocheckboxes example; getting Notice: Undefined index: #ajax in ajax_form_callback()
Reinstall XAMPP! if that doesn't fix it, try a different stack:


Bitnami 
Acquia
MAMP


So installing the Bitnami Drupal stack and testing successfully there indicated a problem with my XAMPP install. Also wanted to try the Acquia stack, was ok, ended up going back to XAMPP.