When you use a Windows computer as a network 'sharing' computer by running Microsoft ICS or other internet sharing software, you must be VERY sure that you do not leave file and print sharing exposed to the Internet.

You should have TWO network connections, each with TCP/IP bound to them. One will be the LAN connection (local area network) and the other will be the Internet connection.

Find your "Network Connections" by selecting "My Computer" from the start menu, then select "My Network Places" and finally select "My Network Connections".

First, Right click the network card that is your INTERNET CONNECTION and select PROPERTIES.  In the picture above, the INTERNET CONNECTION is my 3com connection - I can tell because I have internet sharing enabled so i can see the little the little hand on the icon.  Ensure that only TCP/IP is bound to the Internet connection - we DO NOT want file sharing enabled on connection

Next, Right click the network card that is your LAN network card and select PROPERTIES.  In the picture above, the LAN connection is my wireless connection.  Ensure that all of your services are checked - we want file sharing enabled on this network card.

Next, select the "Internet Protocol (TCP/IP)" and click Properties.

You should now see a screen like the one below.  If you have a network that uses a STATIC IP ADDRESS schema you will see information here.  Most likely, your screen should look just like this.

Select the WINS tab.  Under the "NetBios Setting" area, select the "Enable NetBIOS over TCP/IP"