This is a simple way to drive the average Windows user completely insane. The methods are slightly different for XP and Vista, so I'll go over both ways.
Note: I'm using XP with a Vista theme in most of these pictures.
Step 1: Disable the UAC (Vista Only)
The Vista UAC is possibly the most annoying feature in Vista, and it will prevent our shutdown script from executing if we don't disable it. So let's do that. There are a few different ways, but I think this is the quickest and easiest way. Open Notepad, then copy and paste the following:
Windows Registry Editor Version 5.00
Click File > Save As, change the Save as type from Text Documents to All Files, and save it as DisableUAC.reg. Then double-click the file you just created and click OK when it asks if you want to merge it into the registry.
Now we're ready to write the shutdown script.
Note: If you need to re-enable the UAC later, just change it to "EnableLUA"=dword:00000001.
Step 2: Writing the Shutdown Script
Now for the fun part. Click File > New in Notepad (or open it again if you closed it). If you're using XP, copy and paste this:
shutdown.exe -s -t 30 -c "Your PC will shut down in 30 seconds."
Click File > Save As, All types, and name it Shutdown.bat. Double-click it to make sure it works. To cancel it, you can just Start > Run, type "shutdown -a" (without the quotes) and hit enter.
If you're using Vista, the process is almost exactly the same. You just need to replace all the dashes with slashes. So instead, copy and paste this into Notepad:
shutdown.exe /s /t 30 /c "Your PC will shut down in 30 seconds."
The number after -t or /t is how many seconds the message will stay up before Windows shuts down. You can also use whatever message you want to put in after -c or /c, as long as it's in quotes.
Step 3: Setting It to Run at Startup
We're almost done. Click the Start Menu, go to All Programs, right-click the Startup folder, and click Explore All Users (or just Explore if you want to annoy just that user). Drag the batch file you saved into the Startup folder.
Now whenever someone logs on to the computer, it will automatically execute the shutdown script. It's easy enough to get rid of by booting into Safe Mode and deleting it from the Startup folder, but most people will have no idea what's going on.