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.