Event ID 27 from W32time in sytem event log

In this case XP client machine was member of domain, and its system event log was filling up with warning event id 27 :

Time Provider NtpClient: The response received from domain controller dcname.domain.name is missing the signature. The response may have been tampered with and will be ignored.

The client machine was configured manually with one of the domain controllers as ntp server, and type parameter in HKLM\SYSTEM\CurentControlSet\Services\W32Time\Parameters\Type was AllSync. After changing type into NT5DS as should be ( since client machine was member of domain ) , and restarting windows time service, there was no warning event anymore in system event log. After restarting the time service there was informational event 35 that time service is synchronizing with one of the domain controllers (and it was not the one that was added manually).