If you enable auditing for the Audit account logon events category on Win2K domain controllers (DCs), you”ll see event ID 672.
In the description of that event, you”ll find a field labeled Client Address, which is the current IP address of the workstation from which the user logged on.
If you”re using DHCP, you”ll need to track down the actual computer that leases that address through your DHCP server log.