A system is provided that strongly inhibits infection and spread of computer viruses. Valid executable software files and supporting files, even files provided by mass-released commercial software, are associated with a numeric key that is unique to each individual computer running the software. For a file to be processed by the central processing unit (CPU) of the computer, the presence of a valid key must first be verified. Every valid executable file, including files relating to the operating system and application layer code, is provided with a unique key. Thus, viruses that attempt to gain access to the CPU to perform unauthorized actions, including replication, are prevented due to lack of a valid execution key. Execution keys are generated locally on each individual computer using a variety of methods. Execution keys can be regenerated if the security of a computer system has been compromised, or appears to have been compromised.