List of one ListBox changes with the selection of another

Question

Can anyone tell me or point me in the direction of a tut that shows how the contents of one listbox changes with the selection of an entry from another. For example, I have two listboxes: one for Platform's and one for subplatforms. If a user chooses Windows as a Platform then I want the Subplatform list to relevant Windows subplatforms such as serves and workstations. If a user chooses OS390, then I want the subplatform list to change to relevant subplatforms for that platform. Any thoughts? Thank you. Tim