There are 2 options when installing AC. To install AC on the server, which will be the "main computer" in AC parlance (hosting the SQL Express database), choose the "this will be the main computer" checkbox. For all the other client computers, choose "this isn't the main computer and does not need SQL Express installed".
There is some importance to this. Any computer that has the "full" install of AC & SQL Express can be chosen (accidentally) by one of the clients as the database computer. And you DON'T want two different sets of databases running on your network.
