The Power Supply

The Power Supply

The Power Supply's role is to convert AC to DC voltages. It usually supplies 3.3 volts and 5 volts for the computer's circuits and 12 volts for the motors that spin the disk drives and the fans.

 

The problems that appear with a power supply are usually related to the fan that prevents it from overheating. If the fan doesn't work at full capacity anymore (usually making an annoying sound) your PC will shutdown suddenly or restart with no reason.
When buying a power supply, first consider the amount of power needed by your computer (measured in Watts) and get a power supply that has a greater output than you need. You can also consider the noise that the fan of a power supply makes and the protections it offers (against spikes for example). An example of this is the Active PFC feature.