1st mistake... never use norton, its like one of the worst to use cuz some viruses turn norton into the virus and wont be detected. use avira and malwarebytes they both have free editions. oh and yes i can tell you that you had a trojan on your computer that you could have gotten anywhere. anything you downloaded that wasnt from a well known source (such as a person who has a steam crack/hack) is at high risk for having a trojan in it.