Find out who is logged on to a computer

Download PsTools from here which contains a suite of useful utilities. One of them is called psloggedon which allows you to see who is logged onto a specific computer.

C:\program files\PsTools>psloggedon.exe \\computername

loggedon v1.33 - See who's logged on
Copyright ® 2000-2006 Mark Russinovich
Sysinternals - www.sysinternals.com

Users logged on locally:
     Error: could not retrieve logon time
NT AUTHORITY\LOCAL SERVICE
     Error: could not retrieve logon time
NT AUTHORITY\NETWORK SERVICE
     Error: could not retrieve logon time
domain\xpid
     Error: could not retrieve logon time
NT AUTHORITY\SYSTEM

Users logged on via resource shares:
     11/01/2010 13:36:31    domain\myxpid