Hacked! How do i figure out who it was?

Call your internet company they will tell you, i was hacked 4 years ago, i was talking to that person on my own computer with my own yahoo ID, I saw my files folder were opening and closing, and other stuffs, isnt that scary, so i called local internet company they changed my IP number and port number and changed some other thing then they asked me to install a new window, that will help to stop a hacker, cause without changing this steps they still can come back and bother you.