Download PsLoggedOn. I quote:
PsLoggedOn is an applet that displays both the locally logged on users and users logged on via resources for either the local computer, or a remote one. If you specify a user name instead of a computer, PsLoggedOn searches the computers in the network neighborhood and tells you if the user is currently logged on. PsLoggedOn's definition of a locally logged on user is one that has their profile loaded into the Registry, so PsLoggedOn determines who is logged on by scanning the keys under the HKEY_USERS key. For each key that has a name that is a user SID (security Identifier), PsLoggedOn looks up the corresponding user name and displays it. To determine who is logged onto a computer via resource shares, PsLoggedOn uses the NetSessionEnum API. Note that PsLoggedOn will show you as logged on via resource share to remote computers that you query because a logon is required for PsLoggedOn to access the Registry of a remote system. Just copy PsLoggedOn onto your executable path, and type psloggedon. usage: psloggedon \[-?\] \[-l\] \[-x\] \[\\computername | username\] -? Displays the supported options and the units of measurement used for output values. -l Shows only local logons instead of both local and network resource logons. -x Don't show logon times. \\computername Specifies the name of the computer for which to list logon information. username If you specify a user name PsLoggedOn searches the network for computers to which that user is logged on. This is useful if you want to ensure that a particular user is not logged on when you are about to change their user profile configuration.