Create Access 2000 Combo Box via VBA Mod

I would like to use a VBA module to create my combo box on a form. I would like to base the value list of that combo box on the contents of another control. For example, if Control “X” = “Biological” then the combo box should contain a list of items pertaining to Biology. Otherwise, if Control “X” contains the word “Physical”, the combo box would provide a different list. How can I do that?