When a computer does not turn on at all, there are a few things that can be checked. First of all, check to see if the computer is getting its power from a main power source. Sometimes, there is a fuse that is located in the plug which burns up, and there is no power coming into the Power Supply Unit at all. In this case, replace the power cord immediately. ATX power supplies provide the +5V to the motherboard even when computer is switched off, and it works with only a few circuits on the motherboard. On some computer systems, there is a LED indicator located on the motherboard that stays on when the computer is plugged into a main power source. If there is no LED light indicator, the solution is to check the power with a multimeter. And, don't worry, there is no danger when touching anything in the computer system. Just remember one rule, the power supply must not be opened by an unqualified person since the PSU does have high voltage going through it. Even if the computer system is off and disconnected from the main power source, the risk of electric shock can still occur through the high voltage capacitors located within the computer.
Today, PSU’s are cheap to buy, and easier to replace if faulty, so there is no reason to ever repair them.
It is also important to never damage the PSU or the motherboard when something is not right. Take the multimeter and set it to measure the resistance by 1000 ohms, for instance. Measure the resistance in the socket where the CPU gets power, this is usually the plug with the two yellow wires and the two black wires, and this plug is next to the CPU. If there is a reading for a few seconds and then it goes to infinity, it is then safe to connect another PSU. If there is a reading that is in the tens or hundreds, then it is probably because there is damage that has occurred to the MOSFET which is on the motherboard, and a new power supply can then be damaged if connected. In this case, take the computer system to a technician, or replace the motherboard completely. On the motherboard, there is a circuit that gives power to the CPU, the main components of this circuit are the MOSFET’s. If just one of the MOSFET's fail, the motherboard will not power up. This is what happens when the power button is pressed, a small kick of fans, a little LED flash, and then nothing at all. The PSU protects itself by not powering on.
Even with everything done to make it possible for Vista to run faster, it still does not run as quick and seamlessly as Windows 7 or Windows XP.
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