Internet Access

My company just got an ISDN line and will be giving some employees access to the Internet. The owners are concerned about abuse. Is there a way, on the admin side, of tracking each address accessed by each user? Is there software to do this? I know that I can check each PC’s history and cookies but that is not a good solution. I need to be able to get the info thru the UNIX mail server. Please advise.

Thanks.