BridgeChecker is the type of tool that should have been build into Windows. With this tool you can automatically disable the wireless network interface when your computer is connected to an Ethernet port and enable it back again when Ethernet port is disconnected.
You can however also choose to disable the Ethernet port when a Wireless network interface is connected. Just launch the app, select the Primary Adapter, the Secondary Adapter, and the connection settings. When done, hit Save and it will sit silently in the system tray monitoring the two adapters.
Update: The benefits of using this tool as mentioned by the developer are, conserve IP address allocation, reduce security risks, resolve dual interface routing issues, and prolong battery life.
Note: The default password is 123 but can be changed from Manage settings window.
Right-click the system tray icon and hit Manage. You will be taken to the Manage settings window where you can change the Operation Mode, White-list an adapter, and change the default password.
It works on Windows XP, Windows Vista, and Windows 7 and requires NET 3.5 Service Pack 1. Enjoy!