Try this command from the command prompt:
netstat -a MACHINENAME
where machinename is the name of the machine you would like to know
currently logged on users.
When you shoot this command you will get output like:
COMPUTERNAME <00> UNIQUE Registered
DOMAIN <00> GROUP Registered
COMPUTERNAME <20> UNIQUE Registered
DOMAINNAME <1E> GROUP Registered
COMPUTERNAME <03> UNIQUE Registered
USERNAME <03> UNIQUE Registered
Notice the last line of the output which shows you the username who is
currently logged on.
Hope this helps