Precisely:
start GPEDIT.MSC (just type it into a command prompt session, or at the RUN box).
Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> Security Options.
There you'll find few options that may be affecting the behavior of your machines:

- Network Access: Let everyone permission apply to anonymous users
- Networl Access: Named Pipes that can be accessed anonymously
- Network Access: Shares that can be accessed anonymously

Also, in a different location: Computer Configuration -> Windows Settings -> Security Settings -> Local Policies -> user rights assignment , you may find more settings that may be involved here.
- Access this computer from the network