. . The CPU fan will "twitch" when the switch button is pushed. This will usually happen just once, then needs a rest or to be unplugged from AC for a moment.

One of the most common problems I've seen with inexperienced users doing their own "upgrades" is with unkeyed IDE cables (many are) and one end is plugged in backwards - the power supply sees a short to ground. In this case I'm sure he's tried with everything unplugged.