Facilitating and authenticating transactions

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.