You can pretty much rule out hacking. Hackers generally don't access your computer just to look at porn. If you have a virus with porn pop-ups, these will show up in your registry. Of course, you the most common way to get this sort of virus is by visiting porn sites. So, it may be a combination of the two.
I would advise taking the above suggestion so we can help you with virus problems. And if your kid is looking at porn on the computer, it's best to have a civil talk that doesn't involve yelling or threatening. And of course, installing child safety software tends to help as well.