There is perhaps no feeling more sickening than the one you get when you sit down at your computer, push the power button and nothing happens. No beeps; no whirring start up sound; absolutely nothing. Or maybe your computer has started randomly and spontaneously rebooting with increasing frequency for no apparent reason.

For the average computer user, these scenarios can lead to immediate panic and dread. Something must really be wrong if the thing won't even turn on. You don't know a whole lot about the inner workings of a computer but you've had a peek inside one and with all those parts and cables everywhere the problem could be absolutely anywhere, and that voice in your head whispers "with your luck it's probably really bad".

Thoughts turn to computer replacement cost, massive data transfer, and reconfiguring that home network it took you 3 days to get right. Maybe you can get it fixed. You pull out the yellow pages (since your computer doesn't work and you can't get to trusty old Google), and start looking for local PC repair companies. Sure you'll be without your computer for 2 weeks but what choice do you have?

Chances are you will run into this problem if your computer is more than 3 years old. When you do, the most likely culprit is a bad power supply. It has happened on every one of my last 5 computers. Manufactures are notorious for using cheap power supplies to reduce production costs, but the good news is it is amazingly easy to fix yourself. Once you find the right unit, the repair takes minutes and you'll spend a fraction of the price a technician might charge.

Disconnect all power and external peripherals (printer, external drives, etc) from your computer and remove the cover or side panels. Don't be afraid, it only looks intimidating in there. The power supply is located in a corner. It is probably the largest component inside the computer and looks like a metal box, usually with a fan built in. The power cord plugs into it and there are various cables from the other components plugged into the opposite end. It is very easy to locate.

The box should have some labeling on it that will tell you the power rating. There may be two ratings, a peak wattage and maximum sustained wattage. Focus on the maximum as this is the number that is most important. It will usually be between 300 and 400 watts. It is always advisable to buy excess capacity here. You'll spend a few more bucks, but the component will last longer.

Also, if your computer has a high end graphics card or other high end components, it may have a power supply with a higher rating than stated above. If you've upgraded these components in the past and neglected to upgrade the power supply, this could have caused the failure and you should look at your upgrade documentation to determine how much additional capacity you should buy to properly power those components.

There may be a designation of ATX or AT which indicates the type of case the unit was designed for. Most newer computers are ATX, but there are some older desktop computers out there with the AT configuration. Take note of how the unit is attached and where the screws are. Check the approximate dimensions of the old unit (length/width/height) just to be sure. Finally, whenever I'm replacing something I am not totally familiar with I always take a few cell phone photos before I head out to the store.

Go to your local computer store and buy a replacement unit that at least matches the maximum sustained wattage rating of the old unit. Don't buy the ultra cheap model. It will just burn out quicker and you'll have to replace it all over again.

Once you get it home, attempt to remove the old devise by removing the screws and sliding it out. It's heavier than most parts inside the computer so be careful not to drop it on any of the more delicate parts. If you can get it out with all the connections still intact, do so. Install the new one and then remove the cable connections from the old unit and transfer to the newly installed replacement one at a time.

If you have to disconnect all the cables to get the old unit out safely, remember the trusty cell phone photo. It will help you to easily reconnect all devices to the new power supply once it has been installed. In reality though, most of these connectors are unique in size, can only go into one receptacle and can only go in one way, so there is no need to stress over it as long as you do not try to force anything into where it doesn't belong. Make all your connections, replace your cover or side panels, insert the power cord and boot it up.

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