If Microsoft judges the key as counterfeit or stolen, the usual non-genuine measures kick in: The desktop background fades to a permanent black, on-screen messages remind the user that the operating system is illegitimate, and the PC shuts down every hour.

New PCs come with Windows pre-activated at the factory, so the product key/activation steps are only seen by users who upgrade to Windows 8 from an older edition, or those who use the upcoming "System Builder" to install Windows 8 on a new, custom-built machine.

Microsoft requires a product key during setup because both Windows 8 and Windows Pro -- the two retail SKUs, or "stock-keeping units" -- are included in the same .iso, or disk image, file now available for download from several Microsoft sources. The version installed depends on the key provided.

Windows 8 Enterprise, the version aimed at companies with volume license agreements, uses an entirely different mechanism for activation. Rather than ask the user for a key, Windows 8 Enterprise -- like its enterprise-grade forerunner, Windows 7 Enterprise -- ships with what's known as a KMS (Key Management Service) client key that's activated by an on-premises machine.

Another way to activate Enterprise -- and the method used by copies downloaded from the MSDN and TechNet subscription services -- is with MAK (Multiple Activation Key), which connects with Microsoft's own activation servers.