Question : Any way to see who's logged in?

Is there any (simple) way to see which users have logged into a Windows Server 2003 network and also which workstation(s) they're on?

Answer : Any way to see who's logged in?

Sysinternals has a great little free utility that allows you to check who is logged into a particular machine, or what machines a particular user is logged into:
http://www.sysinternals.com/Utilities/PsLoggedOn.html