Microsoft’s WGA server went down August 24th 2007.
Installations were being flagged as counterfeit because they could not be validated.
Forums.
Topic led by Phil Liu MSFT:
ATTENTION - Vista Validation 0xC004C4A5 Errors - PLEASE READ (Or read the latest post for resolution steps)
Until the situation has definitely been resolved, I would advise against shutting down or rebooting your Vista machines.