The most important thing to remember when troubleshooting a computer problem, is that computers are not that complicated, and there are only so many systems that can break. The real trick to computer repair is figuring out which part needs to be replaced, and taking steps to mitigate data loss.

Turn It On

The first thing to do when running a problem down is simple. All you have to do is turn it on. If it doesn't turn on, check to make sure it's plugged in, and then try again. If something catches on fire, you've found at least part of your problem--maybe all of it. The most common part that can catastrophically fail is the power supply. They handle the conversion of high voltages, and when they fail, they can cause electrical shorts, both internally and in any of the equipment attached to them. Of course, if nothing happens when you flick the switch, your issue probably is not a catastrophic power supply failure.

Strip It Down

If all you get when you turn on the machine is a black screen, the first thing to do is to strip out every nonessential part. Unplug every accessory. When you're done, only four cords should be plugged into your machine: Power, monitor, keyboard, mouse. Internally, only the processor, the RAM and the video card should be hooked up. Even the hard drive should stay disconnected. If the machine still doesn't run, you now know that it's one of those four internal components.

Build It Up

If it does turn on, turn the machine off, add something back in and turn it on again. Repeat this until something causes the machine to fail again. Disconnect the offending device and continue testing. You may see a pattern beginning to emerge. It may be that all USB devices fail, indicating a problem with a USB controller on the motherboard. You might try a different port on the motherboard. Regardless of what you find, test every component.

Replace the Bad Parts

Most PC components are not worth the cost of repair. If a component is still under warranty, send it back to the manufacturer. If it's out of warranty, simply replace it. Most components can only be repaired by an experienced tech; some even require a clean room environment.

Source:

Absolute Beginner's Guide to A+ Certification. Soper, Mark Edward. 2004 Que Publishing.