So the guy who saw your laptop was fired? I'd just deny that you do personal things on your laptop, maybe format if it you need to go that far but if the only person who was your laptop isn't there anymore just deny it.
Don't tell them to **** off if they ask to see your laptop, they'll know your guilty. Remove any trace of anything person and if that require formatting then do it, but let them see your laptop and don't act like you have anything to hide.
Leave it at home, and if he asks about the WiFi tell him you sometimes bring your personal laptop in to work to do comparison shopping on building maintenance supplies to try and save the company some money.