A method and system for detecting an unauthorized client system attempting to connect to a network of computer systems is disclosed. The network includes a certification authority server that includes a database of unique identification codes of authorized network client systems. A client system attempting to connect to the network transmits an authorization code over the network. If the client's authorization code matches an authorization code stored in the database of the certification authority server, then the client is authorized and is sent a network address which the client can use to communicate with other clients. However, if there is no match, the client is unauthorized and is scanned for viruses by an anti-virus server also on the network. Alternatively, the unauthorized client is blocked from the network without performing a virus scan.