Windows Vista Activation error after OS reinstall using Dell OEM KIT

We have a number of Dell OptiPlex 745's and 760's that have been rebuilt using the OEM media kits. The machines are reporting after different periods of time, that the machine has been tampered with and failed license activation.  All are Vista.

• The 1st machine to show this was about two weeks ago and I re-seated all connectors etc, then reinstalled Windows thinking it was an isolated incident.
• 12 days later this machine has again reported the same problem.
• Another machine was rebuilt and within 24 hrs. reported the same.
• To date 4 machines that were rebuilt from their respective OEMS kits are reporting the issue.

The process reports to the Application event log:

• Event ID 1022 - The system has been tampered. hr=0xC004D401

followed by several errors relating to windows std applications

• Event ID 1020 Procy Execution has failed to load.  hr=0xC004D401 Proxy ExecutionPolicy Shell-InBoxGames-SpiderSolitaire-EnabledGame

then

• Event ID 8193 License Activation scheduler (SLUINotify.dll) failed with the following error code 0xC004D401

Has anyone experienced this and found a solution.  The site is a medical practice that sees about 800 patients a day and I am getting a little nervous at these Dell machines suddenly displaying this behaviour.