Carlotta try Diamond or VESTA or Mercury.. they should all be able to visualize polyhedra and I think you can already do something with the free version
Thank you Matteo for the useful advices. Actually, I'm using a trial version of Diamond that allows to build and manage polyhedra (and print images without annoying texts).
Unfortunately, with Mercury it is not possible to do it!
Carlotta, I use VESTA which has lots of nice features, is free and runs standalone, which is what I like most. Another very nice program is jmol, but it makes extensive use of java, and i'm always in trouble with java library versions.
BTW, I'm now looking how a stru drawing can be saved in or converted to a portable vector graphics format. VESTA produces ps, eps, svg which are usu 10x the size of .vesta, but svg is sometimes terribly much larger, I'd like to fix this. Portability is important: for my own and student handouts where I use Latex which sticks to eps, but many editors want you to use ms Office which prefers emf.