It is even ridiculous to think that someone gained access to that PC to surf porn sites .... this makes no sense whatsoever .... I prone to believe that the computer has been used physically by someone to surf porn .... If you said that some one installed a links to porn websites by adding them into the Favorites, I would say that someone could plant a malware or a trojan is playing behind the scene ..... am ought with thoughts ...

But if you are still baranoid .... use the magic command ... C:\Format ...

Cheers