There isn't necessarily a _single_ logged on user in light of terminal
services, fast user switching etc.

That said, this link

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/secauthn/securi
ty/enumerating_logon_sessions.asp

shows how to enumerate all of logons on a system and this link

shows how to get information about each one:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/secauthn/securi
ty/retrieving_logon_session_information.asp

The two links should get you started. Please post follow-up in the kernel
group:

   microsoft.public.win32.programmer.kernel

Regards,
Will