Abstract: This paper discribes the theory and experiments about Detection Expert System of Computer Viruses (VDES) and solves the problem of determining a popular virus in some degree. In order to determine that a given program has been infected by viruses, VDES is based on the rule set formed by knowledge and experience of experts and a forward inexact inference method is used. VDES not only overcomes shortcomings of past checking techniques, such as inexactitude in some case, but also can determine a virus variety, in other words, VDES can determine viruses appearing in the future.