If you have previously installed or registered your Total War: Shogun 2 keycode to a STEAM account make sure that upon reinstall you choose:
Log into an existing Steam account
Once selected you will be prompted to sign-in with your existing STEAM account details that were previously used.
If you have forgotten or do not remember your existing STEAM account details click on:
Retrieve a lost account
Follow the on-screen instructions to retrieve your lost STEAM account.
If you still encounter keycode problems contact STEAM Support for further assistance;
Please be advised that in order to submit a support query you may need to create a STEAM Support account: