What's the Windows Vista reduced functionality mode?
If Vista (and Windows Server "Longhorn," codename for the next Windows Server version) isn't activated within 30 days after installation for the retail release of Vista or within three days of changing a major hardware component, the built-in Windows Genuine Advantage (WGA) feature will place the computer in out-of-grace reduced functionality mode, which will allow access to Vista for a period of one hour. During that one hour, you can perform most functions, including:

• activating the Vista product key
• using Windows Management Instrumentation (WMI) to run scripts on the computer from a remote location
• changing the product key
• using most of the features that are available in Vista
• remotely accessing a shared network location
• running Vista in safe mode for data access purposes

You can't run any of the built-in games or run premium features such as the Aero UI, Windows ReadyBoost, or BitLocker Drive Encryption. If you attempt to log on when Vista is in reduced functionality mode, you'll receive the message: Activate Windows Now. Your activation period has expired and Windows is no longer working. To use Windows you must activate this copy of Windows.
—John Savill