Microsoft’s WGA server went down August 24th 2007, and installations were being flagged as counterfeit because they could not be validated.
Please see this topic:
ATTENTION – Vista Validation 0xC004C4A5 Errors – PLEASE READ (Or read the latest post for resolution steps) in the forums.
Until the situation is definitely resolved, I would advise against shutting down or rebooting your Vista machines.
