This computer must be configured as a domain controller. To prevent this computer from shutting down in the future, run Setup on the disk that you used to install the operating system to configure the computer as a domain controller.
We saw this issue on a network where a small business server domain was migrated to a "regular" domain.
After "dcpromo" was run on the former SBS, we were unable to remove the SBS component anymore, and Windows threatened (yet again) to shut the server down.
The quickest solution is to locate the "SBS Core Services" service in the "Services" application, enter the properties. There, click the "Log On" tab and disable the service for the hardware profile (of which there is usually only one).
The elaborate fix involves suspending the sbscrexe.exe process using "Process Explorer", setting the startup type of the "SBCore" service in the registry to "4", removing all permissions of the sbscrexe.exe for everyone, and killing the sbscrexe.exe process.
Keep in mind that these are all hacks, and a working Small Business Server installation will not generate events similar to the one above.