A wi-fi network is no different to any other network such as ethernet, the only thing is ''it dont got wires'' and on any normal network it is not possible to access other workstations without software running on both that sets up this function, so unless you are a really good hacker I doubt this can be done.
You can just use the microsoft program netsend, obviously assuming that both PC's are running windows. To do this you would need to know either the PC name or the IP address of the machine you wish to send to.
But.... connecting to somebodys WiFi network is illegal, some bloke got fine �500 last year for doing it.