A method and apparatus prevents hacker code from infecting an application program by requiring decryption of the application program prior to running the application program on a computer. The device is preferably a computer system that includes a dongle, or a separate unit that is connected or connectable to the computer. A security program decrypts a first key with a second key stored on the dongle. When a new application is installed the first time on the computer, the security program uses a decrypted first key to encrypt whatever is installed such that the encrypted application program is the only installed version of the application program on any non-transitory computer readable memory accessible by the computer. When a command is given to startup the application program, whatever code is needed for startup is first decrypted using the decrypted first key.