In order for your computer to turn on and run, it needs some sort of power being sent to its components. This is where the power supply comes into play. The power supply is the component that supplies power to all your devices in your computer. From the motherboard the hard drive to the CDROM. Most electronic devices that plug into the wall for power have some type of power supply.
Like everything else in your computer, power supplies don't last forever. You may get many years out of your power supply or you may only get a few months out of it if you are unlucky. When the power supply goes bad you will have to replace it with a new one to get your computer going again.
Here is a picture of a typical power supply. It consists of the power supply unit itself as well as the power cables that connect to the different components inside your computer.
Typical power supply location
Symptoms of a power supply going bad can include random crashes, random reboots or the PC not coming on at all.
When replacing your power supply there are a couple of thing to take into consideration. The most important is getting the right one for your motherboard. Newer motherboards use the ATX form factor and need a power supply with the matching connection. You also need to make sure you get a power supply with enough power to handle all your components. 300-400 watts is sufficient for most home users.
Another thing to consider is extra connections for your devices. Make sure the power supply has enough power connections and the right types. For example you may want to get built in Serial ATA connectors etc. because SATA hard drives can have different types of power connections than IDE drives do. Finally make sure the power supply size is approxamately the same as the old one so it will fit in your case. Some name brand computers tend to have odd sized cases and use smaller than standard power supplies.
Replacing the power supply is a fairly simple job. First of all you need to unplug the power cord before doing anything! Then you can open the case and take the power connections off of the hard drives, CD-ROMs, floppy drive and motherboard. Then you can take out the four screws on the outside back of the case and pull the power supply out from the inside. To install the new one just do the reverse.