After you install a Windows Vista service pack, you might be prompted to activate your copy of Windows Vista, even if you've already activated it. There are two possible causes:
Windows Vista has built in technology that helps Microsoft to combat software piracy. This new technology is a part of the Windows Genuine Advantage (WGA) program, which requires you to reactivate your computer when certain hardware is changed or when certain hardware device drivers are changed. Because Windows Vista service packs might update some device drivers, you might be prompted to activate Windows Vista again.
Malicious software designed to circumvent the Windows Vista activation process might have been installed on your computer. This type of software can alter important system features in Windows Vista and can affect system stability. Service packs include updates that target and disable such software.
To resolve the issue, activate your copy of Windows Vista again. For more information about how to activate Windows Vista, see Activate Windows on this computer.
For more information about how to determine whether a copy of Windows Vista is genuine, go to the Genuine Microsoft Software website.