A computer, such as a Windows-based PC (10), has associated with it a Subscriber Identity Module (or SIM) (12), such as of the type used in a GSM cellular telephone system. The SIM (12) can be authenticated by the telephone network (16), in the same way as for authenticating SIMs of telephone handset users in the network, and can in this way authenticate the user of the PC (10) or the PC (10) itself. Such authentication can, for example, permit use of the PC (10) in relation to a particular application (22) which is released to the PC (10) after the authentication is satisfactorily completed. The application may be released to the PC (10) by a third party after and in response to the satisfactory completion of the authentication process. A charge for the session can be debited to the user by the telecommunications network and then passed on to the third party.