1. A computer monitoring method for monitoring a usage state of a computer based on actuation of the computer by an operating system and on recording of a display image displayed on a display apparatus of the computer, the method comprising: acquiring, when data from the operating system or an application program actuated on the operating system is displayed on the display apparatus, a function to be executed by a subsystem ( 23 , 34 ) that provides a graphics function of the operating system in order to create the display image by acquisition means ( 10 , 35 , 22 ) that uses an input function or an output function of the subsystem; extracting text data from the function by extraction means ( 10 , 22 ); and storing the text data in association with the function by storage means ( 22 , 38 ) in a database stored in the computer or in a server connected by a network with the computer.