What you can do to stop that is to disable remote desktop connections, change your password to a strong one and go to your user accounts and see if any unknown users are added that does not belong there, what could have happened was that the hacker could have gained access to you computer once with administrator rights and created a new user for him with the password of his choice, thats one scenario... The other is that you have netcat on your computer and that opens up a port that the hacker specified, your anti virus will not detect netcat as it is not a virus or a trojan, check if you do not have netcat on...