The quickest way to surely bring down a Windows 9x machine:
  1. Open a DOS console
  2. Start debug
  3. f 0000:0000 ffff ff
  4. Boom!

What happens is that we overwrite the "zero page" (which contains the interrupt table; including the IRQ vectors). Anything interrupt related (including the IRQ0 timer executed (normally) 18.2Hz) will make the processor jump to ffff:ffff, which doesn't contain any sensible code. Windows 9x is unable to recover from this.