When you install ICS (Internet Connection Sharing) on your computer the dial up options are automatically changed so that any computer on your network can automatically dial into the internet at any time while the network and modem are on. If your original dial up configuration was to not connect automatically then this is definitely not what you want happening. Even if you were allowing the stand alone computer to connect to the internet automatically you may not want this behaviour when it is a different computer on the network that is requesting access.
To stop your ICS networked computers from automatically dialing into the internet and running up your phone bill you need to change the dial up setting to "Never Dial".
To do this go into the Control Panel and then into Internet Options. You can also get there by going into the Tools menu in Internet Explorer and selecting Internet Options from there. Microsoft in their infinite wisdom have set uo one place to hold all of the internet related options for their products so even if you are using a different browser you still need to go here to update the dial up settings.
Go to the Connections tab. Here you will find the options to select whether to Never dial a connection, Dial whenever a network connection is not present, or Always dial my default connection. If you choose the first of these then you will have complete control over your network's access to the internet from the computer running ICS as you will have to manually connect to the internet from that computer in order to give internet access to the network.
With ICS installed you will notice that there is an extra button at the bottom right of this screen labelled Sharing. Selecting this button will bring up the ICS options screen. There are two checkbox settings at the top of this screen. The second of these is Show Icon in Taskbar. If you select this option then an ICS icon will be added to your taskbar that you can use to control whether ICS is active or disabled. If you do not wish to completely disable automatic dialing then right clicking on this icon on the taskbar will give you access to enable and disable ICS whenever you need to. With ICS disabled the other computers on the network will not be able to connect to the internet.
This article written by Stephen Chapman, Felgall Pty Ltd.