1. A security system for protecting an IBM personal computer, or IBM-compatible personal comptuer, against unwanted access to the files thereof and unwanted access to the operating system thereof, comprising: an expansion card for insertion into an expansion slot of the personal computer; said expansion card comprising at least one memory means for storing machine code having a first portion for monitoring file interrupt function calls to the files of the personal computer; first means for storing information thereon directed to the access rights of users of the personal computer and access rights to the files of the personal computer; said at least one memory means having a second portion thereof for monitoring all video handler interrupt functions to ensure the security system cannot be overridden during boot processing; and a third portion for allowing communication between exterior processes and the disc operating system of the personal computer through an available interrupt call of the system; and second means for storing said exterior processes for access to said disc operating system via said third portion, whereby access to files are protected by a user-by-user basis, and on a file-by-file basis.