well basicly somehow (I dont really know) the put a file onto your computer and you opened it (probably expecting it to be somthing else, easy mistake.) and with your I.P they can use subseven to control your computer, I've seen the the subseven application and there are litrally 100's of things they can do from deleting files to disableing hardware.
unluckily for me I had to learn the hard way but now I know.
good luck with removing it.