Program to monitor employee internet usage

A member of management has asked me to research available programs to log 
internet usage (web surfing) on our corporate network.

We are running a Windows server with a mix of XP and 2K clients.

What have some of you used to provide this in a report that management can use 
to show that a user is spending too much time on the internet?

I don't have a Linux box up and running, but if the right Linux based app is 
suggested, I would consider going that route.

Thanks in advance.

- Mark A.

P.S. Please try and keep this on topic, I've seen questions like this before 
degrade into a privacy violation, "big brother" type of debate.

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