Use a trusted server to communicate with the App Store.
Did you run the networking wizard once you reformated and reinstalled the OS.
If you are telling me that you are using WPA with a shared key on your router, then you are using WPA - Personal.
You want to make sure you are using that option on your computer.
You just have to find the common setting between the two devices.
When setting up WPA you will see the option for either a radius server (Enterprise, and you see this in the corporate environment) or a place to enter a passphrase (Pre Shared Key, PSK).