The fact that a computer shows up in shared does not indicate that you are sharing anything. It means someone else on your network has file sharing turned on.
If you are using a cable internet connection, then there is a possibility you will see other computers on the cable network. I'm not sure about other types of connections. If you have an unsecured wireless network, then other people could be using your network; sometimes it is accidental, other times not. WEP is also not secure.
As for permissions, if you don't give out usernames and passwords to random people that pop up on your network, they won't have access. If you have Guest access turned on, and you have file sharing turned on, then anyone on your network can access public folders, but nothing else.
All the permissions as they are set are very secure. Changing them will not make your computer more secure, but it will likely cause serious problems with its function.