How do I create a modular product build system in a shopping cart?

Hi
We sell childrens climbing frames and i'm trying to find out how to build a system or buy software that can be used to "build your own climbing frame" system. When someone has picked there base tower, they then can choose their slide, drag and drop and the cart adds the total. Does anyone know of software that does that sort of thing? Any help appreciated.

I would begin my search looking for AJAX applications, they are very powerful and have that capability