If you have installed on your system File-and-Printer Sharing and use your system also to
connect to the Internet, then you need to make the following change to your configuration:

Select the TCP/IP-protocol communicating via the Dialup-adapter (which is used for connecting to the Internet) and display the properties:

You will be warned, that TCP/IP settings should be changed as part of the Properties of
a Dialup-connection, but there is ONE issue, which you can only configure here.

In the TCP/IP Properties, select the tab: Bindings By default, the protocol will be used to control outgoing network connection (via the "Client for Microsoft Networks") and incoming network connections (via "File and Printer sharing"). While you like to establish an outgoing connection to the Internet, usually you do NOT like anybody on the Internet to connect to your computer to the resources shared by you (which for home-networks are usually without any password protection). To avoid this, do NOT allow incoming Network connection by un-checking the Binding to "File and Printer Sharing"

If you like to allow somebody to connect to your disk (like a friend), then you MUST define during Sharing of a Disk or a Folder an "Access Password", which the person attempting to access your shared resource will need to know.

This is NOT a joke : One of my friends is connected to the Internet via a cable-modem,
and while he was browsing the Internet, his printer started to work: It was a message from
a "good" person on the Internet, advising my friend that his system was wide open for
access from the Internet (and if the disk was shared with "Full" persmission as Access-Type, that includes
the possibility to delete every files on your disk, if your visitor is a "bad" person).

The above is only a first step, offering a first low level of security.
You may want to test the security of your system and then install a Personal Firewall.