Windows 7 Ultimate x64.

The TrustedInstaller service is failing to start with an error about a file missing, and sure enough `C:\Windows\servicing\TrustedInstaller.exe` is missing.

What is the easiest way to restore this file? I can't run SFC from within Windows, because that results in the error "Windows Resource Protection could not start the repair service"... which happens if the trusted installer service is not running.