Security problem?

I have a home network connected with SMC 5616DS EZ Hub 10/100, which is connected to AT&T Plug&Share Router. Network is connected to the internet via cable modem. Router firewall is disabled. Every PC is behind Zone Alarm firewall. Today, I noticed another network under Microsoft Windows Network. I don’t remember setting up another network (especially with the name “4”). “View workgroup computers” shows only computers that are on my LAN. Is there any way to delete this network? Also, i’m wondering if someone hacked into my PC (how do I find out for sure?)?
Any help on this would be greatly appreciated