A method of adding a copy protection function to a program to be installed on a computer system comprising a copy protection unit is provided, said method comprising the steps of determining a decision section of the program, wherein information influencing the further course of the program is defined during execution of the program, as a function of the current running state of the program, and converting the decision section by means of code which is executable exclusively in the copy protection unit and which is executed in the copy protection unit during execution of the program.