Convincingly "breaking" a system for a non technical user

The command line interface for the DiskCryptor Open source partition encryption software includes a -bsod parameter, the wiki says it will

Erase all keys in memory and generate BSOD

(emphasis mine).

You can use Windows Task Scheduler to schedule its execution at a given time, and it should generate a BSOD as desired. I have not tested this solution, as I am not currently on Windows, but it is stated to work in 32 and 64 bit versions of Windows 2000, XP, Server 2003, Vista, Server 2008, and 7.

But don't stop at that:

You can use this reboot tool to display custom cryptic-looking messages that ask for a reboot and perform it after X seconds. Set X to a really low value. Set multiple events in the Task Scheduler. Alternatively, use a reboot tool that will display no message, or the builtin shutdown function.

Finally, there is also NotMyFault

Notmyfault is a tool that you can use to crash, hang, and cause kernel memory leaks on your Windows system. It’s useful for learning how to identify and diagnose device driver and hardware problems, and you can also use it to generate blue screen dump files on misbehaving systems.

An AutoHotKey script I often used during dorm parties to stop drunk people messing with my PC while keeping it on for music.

a::return
;...
; repeat for other lowercase letters
z::return
A::return
; repeat for other uppercase letters
Z::return
1::return
;...
0::return

f1::return
;...
f12::return

LButton::return
RButton::return
MButton::return
#::return
#r::return
Delete::return
Enter::return
^!Delete::return
^+Esc::return
Space::return
LWin::return
!f4::return
Up::return
Down::return
Left::return
Right::return

;hold appskey and press z and 9 to unlock
;picked those three because they are far away from each other and unlikely to be pressed simultaneously even when buttonmashing
appskey & z::
  If GetKeyState("9","P")
  Suspend
  ExitApp
return

Put it in autostart and the computer will not react to any input (all buttons are remapped to do nothing, all you can do is move the mouse). The only way to fix it is to restart Windows in safe mode with autostart disabled or by knowing the secret combination.

There is another way by using:

f11::blockinput,on
f12::blockinput,off

But it might require admin privileges (and therefore can be simply chosen not to be executed when prompted after a restart) plus it does not disable Ctrl+Alt+Del.

How about BlueScreen? This screensaver was running once on a PC of a colleague. The crash was so convincing that I resetted his PC while he was away. I thought I do him a favor by not letting himself wait for the boot process :-)

Running Notmyfault /crash should also generate a BSOD. Be sure to configure Windows not to reboot, so that it actually shows the blue screen.

This piece of C++ code runs 8 threads in realtime priority, so it should cause a 100% CPU hang even on modern i7 processors. You can do nothing but press the reset button.

void WasteTime()
{
    int priority = 15;
    ::SetThreadPriority(::GetCurrentThread(), priority);
    while (true)
    {
    }
}

int _tmain(int argc, _TCHAR* argv[])
{   
    ::SetPriorityClass(::GetCurrentProcess(), 0x100);
    for(int i=0; i<7; i++)
    {
        LPDWORD threadid = 0;
        ::CreateThread(NULL, 64*1024, (LPTHREAD_START_ROUTINE)&WasteTime, NULL, 0, threadid);
    }

    WasteTime();

    return 0;
}