Tracking “deleted” websites

After finding our home computer filled with some gross porn, I confronted my brother and asked him to please stop loading our system with all the cookies and possible spyware invasions. He seems to have complied–or else he is regularly deleting all his temporary files & cookies. Plus I just noticed he has made his User folders inaccesible! How can I monitor what he is doing? We both have admin privileges since we bought it together. I feel like he is always a “step” ahead of me! He uses both Mozilla & IE.