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).
Step 2: Place the File
Go to the control panel and look at the icon that says "Programs". Under it should be some links, one saying "Change Startup Programs". Click on that one. From there it should be pretty self explanatory. Just add the file to the list.
Step 3: Preparing to Deploy
Keeping that in mind, we will open up Run on one of the computers. You can do this by either hitting both the Windows key (the key with the Windows logo on it) and the "R" key, or by going under "Accessories" in "All Programs" in the Start Menu. When the Run bar comes up, type in "services.msc". (Of course, without the quotation marks.) Hit enter. A new menu should come up with a list. The list is in alphabetical order, making navigation easy. Find where it says "Messenger". Right click it and select "Properties". Where it says "Startup Type", put "Automatic". Exit out.
Repeat this for all the computers. You'll get fast at it with practice.
To check if a computer already has net send enabled, open the Command Prompt. It can be opened either by typing "cmd" into Run, or by clicking on it in "Accessories". Now type "net send /?" (again, without the quotation marks). If it responds with information about the command, then it is already enabled. If it responds by saying that "net send is not a command", than net send is not yet enabled.
Step 4: Deploying the Freeze
Watch as everyone's computer starts swarming with pop-ups. Soon the tech department will be called, but the only thing to do will be to shut off the computers. This would normally stop it, since shutting off the computers would mean shutting off the program, but because we put it in the startup folder, it will start all over when the right computer is turned on. It is a brilliant prank, as the problem computer(s) could be one of many identical computers. Again, have fun!