I was out looking for some sample scripts at Script Center (http://technet.microsoft.com/en-us/scriptcenter/default) a nice place to look for scripts, everything from VBS to PowerShell and they have all been categorized as well!
Anyway, some customers asked for a way to disable wireless connection when LAN is connected. Some vendors have an option in BIOS and some have an application or a service that can be installed.. If this is not the case for you, this script could help you, tune it to get it to work for you and create a scheduled task to disable WiFi
When creating the scheduled task (In Windows 7) you can create a trigger for events i.e. in Microsoft-Windows-NetworkProfile/Operational, don’t forget the option in the Conditions tab: “Start only if the following network connection is available”