I can remember using a dos command that returned the logged on user at the time when I have entered the computer name - however I have searched the net and can't find the correct command.

Please help:)

This is not always perfect but the one you are talking about is NBTStat. It returns the entries in the NetBIOS translation table statistics for a given computer. You can either do so with the IP address or the computer name:

NBTSTAT -A xxx.xxx.xxx.xxx or
NBTSTAT -a ComputerName

NBTSTAT /? For a list of all of the switches you can use.