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 method includes steps of: providing a security device that is a separate unit from components necessary to operate the computer; storing a symmetric private key on the security device; using the device symmetric private key to produce an encrypted application program upon first installation; thereafter decrypting that part of the encrypted application program needed implement a command to run the application program; and, decrypting, on the fly, only those follow-on parts of the encrypted application program needed to perform functions called for during operation of the application program.