A computer system includes a CPU (central processing unit), a main memory, a BIOS ROM (basic input/output system read only memory), a display device and an input device, and using an OS (operating system) program having its product key. The computer system further includes an OS program storage storing the OS program therein, a product key storage storing the product key of the OS program, a product key storage program reading the product key of the OS program stored in the product key storage to be inputted, when a product key of an OS installation program to reinstall the OS program and the product key stored in the product key storage are identical. With this configuration, where an OS program is to be reinstalled due to damage of a hard disk drive or a critical error of the OS program, the stored product key is read out and is automatically inputted into a product key input window, thereby giving a user convenient computer environment.