Stamping your Windows Embedded Standard 7 image with a product key is essential, if you and your customers want to have have a pleasant experience with the device you are building. If you do not stamp the WES 7 image will be an evaluation image that stops working correctly after 120 days. After this it is going to reboot every 20 minutes.
To avoid this boomerang effect, you should put the PID you get from your distributor into the OS image configuration in Image Configuration Editor (ICE).
The right place for it is in the setup feature pack below Product\ Operating System\Foundation Core\Windows Embedded Edition\Shell-Setupx86.
The setting is called Product Key. Just past the PID in there and re-create the unattended.xml file.
The PID, which – of course – is sensitive information, is included in the file in plain text, therefore have an eye on the file to avoid it getting into the wrong hands.
If You are using Image Builder Wizard (IBW) you will get asked by the wizard to provide the PID during the installation process.