This video needs to open your eyes to the risks in any network. A simple application called Cain and Abel poses great risk to your network. If a person installs this app on a company machine or connects a personal computer to your network you are in big trouble.

The days of allowing all your users to be members of the administator group just for your conveniance so that you do not have t troubleshoot poorly written apps that do not run well under the restricted user account are long over.  Allowing users to be administrator so that you do not need to spend time installing software will cost you.  The best practice is forcing them to use restricted accounts, part of the users group.  Please remember never allow users to have admin access to their work computer. Also this should reveal the need for port security on your switches and wire security in your network.