Showing subcategories only?

I'm working on customizing a ZenCart site and I have hardcoded the main categories into a top navigation bar (since they are unlikely to change and we wanted to do it graphically). What I am trying to do is make a side navigation that displays the sub categories (and beneath that, the sub-sub categories, and so on down) for just the main category that is selected. I do not want the main categories to show up in this side-nav. Does anyone know if this has been done or if there is an easy way to do this? I apologize in advance if this is in the wrong place or if it is a silly question.

Thanks,
Matt

Your question isn't silly- but I have a question for you - why would you hard code the top level categories when you could use the built in - dynamically generated top level cats using the Cat Tabs? You could have added your graphics to it.