I was taking apart a computer. I opened up the power supply and I was told off because it was Dangerous.
Modern computers use switching power supplies that do not have large capacitors and the largest voltage difference between outputs is 24V which isn't enough to hurt people. The main danger of working on an energized computer is if the main power is somehow shorted to the chassis. A computer that isn't plugged in is not dangerous. Now if you have an old vacuum tube TV sitting around voltages and capacitance in that baby can be high enough to kill you. One of the practical jokes we played in electronics lab was to bend the leads of an electrolytic capacitor out and charge it to a stinging but harmless voltage then quickly toss it to an an unsuspecting classmate. Electrolytic capacitors can store quite a jolt but since they discharge quickly you only have a few seconds before they are powerless.
Because there are capacitors in it which pack a hell of a punch ad the charge they carry is very dangerous if it is discharged accidentily