You could try to do it with buttons. You can convert any image or pdf to a button and tell the button that another button should be visible on rollover, that will open the first button, and the second button will have an action connected if you click somewhere in the whole area, you can also make several buttons visible and unvisible with one click.
I think it could be accomplished with interactive PDF if you chose Show/Hide fields.
Do a Google search on "show hide fields InDesign tutorial"
BTW, only the actions at the top and at the bottom of the Actions list in the Buttons & Forms panel can be used in PDF files. The ones in the middle are only for Flash (SWF).
Thank you for your help Willi & Steve. After some trial and error I have got the document functioning just how I had planned!
It took me a while to work out the show/hide function to make boxes disapeer once another topic was selected, but in the end this was very simple using the action view on/off options and hide until triggered.
Many thanks again