Hello Giuliano,
Along with the recommendation of dynamips, I would suggest downloading gns3, which ties into dynamips. You can run the same version of IOS that you are working with in production, and there are versions for Windows/*nix.
It acts more like an emulators at first glance, and does not seem to have the same limitations as some of the other simulators out there. Just make sure you have the hardware to support it.
Thanks,