LAN SAFETY 

When you access the Internet from a computer that's attached to a
network, you need to turn off file- and printer-sharing while you
surf the Internet. The problem is that when you're connected to the
Internet while on an active network, you're making the entire network
available to the ISP. 

Although the chances that someone will access your network are small,
there is still a chance. The best approach is to turn off the sharing
before you log on to the Internet. Microsoft Internet Explorer 3 will
prompt you to turn off sharing unless you've already told it to stop
issuing the warning.