Windows strange exploit maybe?

So a friend of mine talks to some shady assholes on skype and somehow they are able to make his PC unable to boot, they did this with his first machine running windows 7 a few months ago and then to his machine running windows XP. Im not sure if its that the machine wont boot duo to the boot device being changed or the system 32 has been damaged or something. This all seems to be done remotely and he has not accepted anything from them anyone have any idea what mite be going on and how this keep this from happening and how to prevent this?