For something like this, you might consider another way to enter the choices. I have never been able to figure out a satisfactory way of doing what you are trying to do with check boxes. My suggestion is to try it with two list boxes. Have one that shows all the choices, then show one that shows what has been selected. It is similar to the List Box wizard. When you are telling it what fields to include in the List Box (when it is based on a table or query) there is a list of available fields and a list of selected fields.
It can be alot of code, so if you aren't comfortable with VBA then it might be difficult to implement. But for me it has worked for the situations I needed it for.