You may end up having to call in to finish the activation. It's all automated and not a big deal.
After you enter the code given, take a picture of your screen before hitting next.
The same code will work on the same computer next time if you need to reinstall.
Although not needed in this case, I've used a program called magical jellybean to retrieve product keys.
My friend had a hand me down WIN 7 PC from his father, but didn't know the password and there was no product key sticker.
I was able to retrieved the key by attaching the HD to my PC and using magical jellybean, then was able to do a fresh install of Windows.