Go into /Library/Preferences/System Configuration/ and delete:
com.apple.airport.preferences.plist
com.apple.network.identification.plist
com.apple.smb.server.plist
NetworkInterfaces.plist
preferences.plist
You'll then have to add the Network Services you want to use in Network System Prefs and configure them appropriately. Just by adding them back you will create the default setups for them.
NetworkInterfaces is the list of what Apple calls Network Services in the system prefs, like Ethernet, Airport, etc.
Network Identification is all of the networks you've ever connected to.
Airport Prefs are all the wifi networks you keep in the preferred or recent list.
SMB is the NetBIOS setup for Windows file sharing on your Mac (name, workgroup, etc).
preferences is the Network Locations and associated setups for the services.
If you have com.apple.nat.plist, that is the internet sharing setup.