Load Virus;
Reset CMOS;Destroy Registry;Delete Drivers;Rename System Files;Invalid boot.ini,.....;
Use a bootable floopy disk or CD to mess up the directory;
Short-circuit VGA connector,USB Port, keyboard/mouse ports...(you only need a piece of wire)
If you can open the case, you can do anything...
Well, the deadliest one, if you know how to flash bios, that will be too easy to destroy any computer. Make a backup copy of the bios to a floppy and edit it(change any few values) with Winhex and save it, then flash it back to your computer. Restart the computer, it's dead!
Got it?