on the server, control panel/server/user
it shows user name, computer, resources opened by user, time, "ocioso" (sorry I am usin a portuguese version, and don´t remember te word in english) and if the user is a guest (but It don´t make any report if the user goes out it will be cleared).
Else you can use event viwer. At user manager, policies/"auditoria"(the last option) mark logon and logoff. It will be registred on event viwer log/security, all logins and logoffs. You need to see in PDC and all BDCs, about this event, because any of this servers can authenticate a user in login.