I didn't upgrade though. I bought a preconfigured computer and it came with Win 8 already installed.
You can't do that then. Prebuilts come with OEM licensing. They are valid for install on one PC ONLY and are locked to the motherboard. You are not allowed to change the motherboard, as that constitutes a new PC, unless it fails and can't be repaired/replaced with an identical one.
You have to buy a new copy of windows. the upgrade assistant shouldn't even work with an OEM key.