How do I find out who is logged into Windows XP?
You can see logged on users in the Users tab in Task Manager.
qwinsta from the command line should show you who logged on, though I'm not sure if that's just for terminal server sessions or not.
C:\Users\wumpus-home>qwinsta
SESSIONNAME USERNAME ID STATE TYPE DEVICE
services 0 Disc
>console Jeff 1 Active
psloggedon - a small utility that is part of the SysInternals Suite of tools
Stand-alone link: http://technet.microsoft.com/en-us/sysinternals/bb897545.aspx