Connect to other computer through Wireless

Hello, dear Geeks .. :)
Two computers are connected to a wireless network .One is connected by a cable network to the modem ,and the other is using Wireless ....
My question is that ,Whether it is possible to connect to(hack into) the laptop without permision(Even by knowing the Wireless Password Permission) ???? Or vice versa ,laptop to computer ??????????

it's TOO important for me ... I think my friends is Cheating in my computer here in my work place .... PLEASE GUIDE ME ....

Thanks a lot :)