One machine, one router, multiple users – need help

Hey guys, hopefully you’ll be able to help me with this.

We have an XP machine with multiple user accounts behind a Netgear WGR614 router, and would like to know the exact time each user on the machine logged on/off. Is there a freeware application or script that could log this information on the router?

Please feel free to ask for clarification if you need it. Thanks.