Two options that I can see... pick through your web professor's code:
http://www.csrees.usda.gov/scripts/triggerParams.js
http://www.csrees.usda.gov/scripts/stdLauncher.js
Or you can pick through my code (a mix of Javascript and HTML layers. Then benefits to doing it my way is that each "button" for the drop downs can be assigned an images and is HTML controlable.
View Source:
http://www.swsphoto.com (IE only)
http://www.wnng1350.com
http://vidaliaonion.org/ (vertical menu bar instead of horizontal)
That should get you started.