This mass freezing will work by having one or more "host" computers send hundreds of thousands of pop-up messages to everyone in the network. Every time you try to click on something, more pop-ups have gotten in the way, soon freezing the computer. When all of the computers are shut off and rebooted, it starts all over!
Step 1: Write the Script
net send * messagehere
What this will do is send a pop-up message to all of the computers in your network, and then loop back and do it again. It will go as fast as possible, taking up all of the CPU usage. That way, no one could stop the program (as that takes up CPU) without turning off the machine. As all of the network's computers are going ballistic with pop-ups, no one will know which one is causing it (especially if you launch this from two computers in the network, as they would both be causes but look like victims).