It is the purpose of a Firewall to prevent others from accessing any "shared" folders. This is why it is necessary to enter the IP addresses of networked computers in a Firewall's "trusted" section so that those computers *ARE* allowed to access "shared" folders over the network.
If not already using a third-party firewall, such as Zone Alarm, I would recommend doing so in order to control very easily access to the PC and which programs in the PC can go online to the internet.
If you don't have a network of two or more computers then, in the Properties of your network adapter(s), the "Local Area Connection" and/or "Wireless Network Connection" untick the box "File & Printer Sharing for Microsoft Networks".
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