"I have two combo boxes (or list boxes) on my form. When the user picks an items in one list I want the choices in the second list to be narrowed down".
This remains one of the most frequent questions Access database users run up against early in their work with Access forms. It is sometimes referred to as cascading combo boxes, or dependant combo boxes, or synchronizing combo boxes. There are many good posts on the subject on the Internet. I posted on this back in November, giving a couple of good links then, including a visual how-to from Microsoft and Candace Tripp's frequently cited examples.
This morning I ran across this post in the Microsoft Knowledge Base. It has a very straightforward solution which I use in most of my forms. It's the same approach that's at the heart of Candace's examples, but the Microsoft post is very concise in describing the technique.
The Microsoft post also includes a downloadable database of example forms that I found useful when I was learning to develop forms. Of interest is that the combo box example if the database uses a different technique to "synchronize" the combo boxes. It uses the "almost no code" solution where the query that feeds the second combo box is built to use the first combo box as a filter. This is probably the easiest approach for someone who's not a VBA programmer.