Just say it was a porn ad. There's nothing you can do, except to reinforce when they are 'investigating', and ask "How long was I on that site? The box popped up when I visited a site I thought was legit (Even if you didn't), and then I closed it right away."
I work in IT, and I'll tell you porno sites leave a ton of login cookies, etc. Most anyway.
Do you sit in an office with others? If so, I mean Hell, why would anyone sit in the middle of an office in their cube and surf porn and risk the chance of being caught?
You'd be surprised how many do, but you can also play that angle. I think they are going to fuck you if you don't bend it a little, because they are going to believe what they want to believe, Star.