Simple, remove your heatsync and fan then overclock your CPU by 100%. If you don't cause a fire, that'll ruin your PC.
Here's much easier (and safer way) to ruin your PC. Turn it off, unlpug everything from the outside and release a static shock on every component you want to kill. Use it on the CPU, mootherboard, RAM, videocard, if it's goign to be replaced, shock the shit out of it. Wear wool shocks and shuffle your feet, then make contact with these parts without your body touching anything else. The electrostatic discharge will ruin the RAM for sure, making your PC unbootable.
Wikipedia article.
Jam it back in, in the dark.