Tracking Internet use with Exchange 5.5

Looks like I’m going to become a snoop, but my boss has asked (told) me to track the web sites that each of my users (42 of them) visits. I know of the urlcache folders, but they only show me the “Where”, I also need to know the “Who” and the “When”. Is there a way of logging web use within Exchange or will I need another bit of software?

Thanks

Steve