1. A computer software security system for restricting execution of a program to a particular machine comprising: first storage means for storing a machine identification code unique to said machine; second storage means for storing a machine identification code in said program; means, responsive to said second storage means, for determining the presence of said machine identification code in said second storage means during execution of said program; means, responsive to said first storage means, for retrieving said unique machine identification code from said first storage means; means, responsive to said means for determining and said means for retrieving, for comparing said unique machine identification code from said first storage means with said machine identification code in said second storage means; and means, responsive to said means for comparing, for preventing further execution of said program unless both said machine identification codes are present and match.