"I have two combo boxes on my form. When the user makes a choice in one I want the choices in the other on to change. For example the user will pick a country in the first combo box and the second combo box will now show only cities in that country. How do I do that?"
That's commonly called Cascading Combo Boxes and is one of the most commonly asked about techniques. I ran across a new article on this topic the other day. This a blog post from the Microsoft Access team that points out two different ways to get an Access 2007 sample database that demonstrates and explains two simple approaches to cascading combo boxes.
See also: all my Cascading Combo Box posts.