I do not think that you are more expose to attacks. but some thing is wrong with your network configuration.
The two network phenomenon usually occurs if there are computer with workgrouips name that are different than others.
Or if any of the computers have a Network manager (like Netmagic, or Bonjour), or some 3rd party security suits that like to have its own way with your Network.
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My position on peer-to-peer home Network.
Never use more than one active software Firewall/AntiVirus/Spyware.
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The differences between the top security suits are minute.
Each one might miss something when tested, but in general they provide similar protection.
As long as a person uses a single computer connected directly to the Internet it does not matter much which one is used.
The problem of using 3rd party security suits stems from their difficulties (lack of flexibility) to handle variety of Networking situations.
If you spend time on Computer's Networking Forums you will amazed by the number of users that in order to have a functional LAN and Internet connection have to Uninstall the 3rd party suits, and clean their computers from the residual running processes of the security suits.
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The First level of protection with Broadband Internet connection is using a Cable/DSL Router, even if the Network is comprised of one computer. The Router's NAT Firewall & SPI providing very good inbound protection. The Routers WAN side also takes care the ISP connection's authentication, thus keeps the connection always alive.
On each Networked computers running Windows Vista/Win 7 use the Advanced native software Firewall, and install the free Microsoft Security Essentials as AV and Antispyware applications.
http://www.microsoft.com/security_essentials
It all working smooth, on both Cable Internet and DSL, maintaining very good Internet/Network security, without adding any cost to each computer.
I configured many computers on peer-to-peer networks with security configured as described above, many of them are Giga LANs, and they all working very well and secure, achieving LAN transfers up to 70MB/sec. (B=Byte).
The only time that I deviate from this, routine is when it is essential to get Visual feedback during Internet/Network activities and actively control any application that tries to connect to the Internet.
In such cases I usually disable Windows native Firewall, and use PCTools Firewall, it is free, small, and minimally intrusive Software Firewall. The download is only 10MB, and the active processes in RAM use about 14MB),
http://www.pctools.com/firewall . It flexible pop-up and traffic indication helps to efficiently control the allowed connection list.
