Internet Usage Help in server 2000

I hope this is in the correct place. I am new to posting here.

I work at a small company that has roughly 25 computers hooked up to a server running server 2000. The person who set up the entire system recently left (around six months ago). I have been for the most part in charge of the network because I have the most computer experience, but very little server experience.

Now my question. Management here suspects someone of surfing the web and looking at porn all day. I know this is a vague question, but is there any way of knowing what they are visiting and what machine it is going to, that would hold up in a court of law? They do not want to confront the person without proof, and have asked me to provide the proof. We have an ISO machine we are using as a firewall, but everyone has unlimited access to the web.

As I am sure you can tell I am a newbie to server 2000 and any direction or help you can give me would be greatly appreciated.

thanks.