The best option would be making an image of your harddisk and giving it to an expert who can figure out whether your computer is infected with a worm, virus or trojan horse. If that's the case there might be some evidence left that could lead to the culprit. Unfortunately, in this sick world, the police might confiscate your complete computer and possibly even do a house search. If there's really such illegal material on your computer, it's you who might end up in jail whether you're guilty or not. Even if you don't, you neighbours might beat you to a pulp or even murder you once they hear about this case.