The present invention relates to an authorized operating system environment and ensuring that a computer is running the authorized operating system environment. The invention provides a method that improves the security of virtual machines. A computer operating in the authorized operating system environment is adapted to send notification signals to a verification server. The signal comprises information identifying the computer. In case the verification server fails to receive a valid notification signal from the computer within a specified time, the verification server will provide a fault indication. The invention also provides an associated operating system environment and other aspects.
