To-do lists on small office network

We have a small office network of one server and 5 computers / workstations. We want to find some software to share to-do lists and possibly calendars.

Does anyone have experience of a basic software solution? The objectives are to be able to allocate and view other peoples to-do lists and maybe enter diary appointments etc if possible. We currently use outlook but are told we cannot link each machine up because our network is not sophisticated enough. The calendar option is a low priority, but the share to-do lists would be excellent.

Can anyone help please?
Simon Laskey