If they are all locking up at the same time, then the problem is likely to be in any common hardware/software - which would most likely be either the networking hardware or the server.
This begs the question - have there been any changes to your network switches since the last time it all worked properly? For example, if you are using multicasting when sending out images to your desktop or laptop machines, you need to have IGMP snooping enabled on the switches.
When the machines lock up, are there any pertinent entries in the locked up machines event logs, and in the server event logs for the time of the lockup?
If a Ghosterr.txt file is being generated, then indeed you should post it here, as requested by Nitin above.