Why not use remote app? That way users just click the Sims shortcut and it will run the Sims app and not the desktop. Stops you having to bugger about with locking the server start menus, desktops etc down.
We have our Sims setup so that it will logon using the users credentials. Makes life so much easier and I don't have quite so many teachers doing dumb things like writing their username and password down on a piece of paper and leaving it on their desk.