Computers, which are built by large manufactures that come with Windows Pre-Installed, come with two (2) Product Keys:
OEM SLP: This key comes pre-installed in Windows, when it comes from the Factory. This key is geared to work with the OEM Bios Flag found only on that Manufacturer's computer hardware. So when Windows was installed using the OEM SLP key (at the factory) Windows looks at the motherboard and sees the proper OEM Bios Flag (for that Manufacturer and that version of Windows) and Self-Activates. That's why you did not need to Activate your computer after you brought it home.
COA SLP: This is the Product key that you see on the sticker on the side (or bottom or sometimes in a laptop's battery compartment) of your computer. It is a valid product key, but should only be used in limited situations (such as this case when the OEM SLP key stops self-activating for whatever reason). The key must be activated by Phone.