Your IT department (if it exists) can do it without any additional software using nbstat. If they write a script for all the PCs they can tell and any given time who is logged on or not.
http://www.windowsnetworking.com/kb...ips/accounts/nbtstatrevealswhoisloggedon.html
Aye just saw nbtstat pop up, not tried it yet but seems pretty useful and painless