Are you running an Active Directory domain in this lab? If so, run the 'net time /setsntp:tock.usno.navy.mil' command ONLY on the domain controller that holds the PDC emulator role in the forest root domain (first domain that was created). All other members of an Active Directory forest will follow an algorithm for syncing time with that one DC. Do NOT sync time on each client and server, or you will be in a world of hurt.