PC's are actually much easier to repair these days than in the early 90's when I wrote the my old guide for technicians I was training. The number of discrete parts in a PC has dropped radically since I first cracked open an PC AT in 1985. The average PC these days has less than a dozen parts, unless you start counting cables, and that total includes the keyboard and mouse! So, some new techs figure the way to learn computer repair and hardware troubleshooting is to carry around a few spare parts and swap-til-you-drop. Well, it doesn't really work that way for a number of reasons, including the fact that all the really tough problems are intermittent, so diagnosing the problem correctly is actually the main challenge. Computer hardware problems are less common than software problems (just think about all the malware and viri running about the Internet), and there are far fewer variables to consider when learning to troubleshoot PC hardware. The real trick is to go about it in a systematic matter, eliminating possibilities whenever possible before you start purchasing replacement parts. To that end, I developed a series of diagnostic flowcharts for logical approach to computer repair. Four miniature flowcharts are below, the full size versions are accessed by clicking on the images or the links. All of the flowcharts are excerpted from the published book "Computer Repair with Diagnostic Flowcharts" which is also used as a technicians field manual and a course text in technical colleges.Edit
Click on the thumbnails below for full size interactive troubleshooting charts.Edit
The first place to start is always confirming that the power supply is operating properly. When you start troubleshooting a dead computer, never ignore the possibility that the AC power to the PC's power supply is at fault. Power supplies often fail gradually, giving rise to symptoms that appear to be caused by individual component failure. Many cheaper PC's ship with power supplies I would basically describe as "disposable." If I had to choose one part to blame the majority of intermittent failures in cheap PC's on, it would be the power supply.Edit
When we talk about troubleshooting video failures, we're usually talking about no image at all on the screen. The easy cases to diagnose are those where the monitor or LCD isn't powering up properly, or the PC not powering up. Video card failure isn't uncommon, and video cards can lose their contact with the motherboard, especially early AGP adapters which frequently popped out of their slot. Video failure can also be due to motherboard failure or to external interference, when it comes to poor image quality.Edit
There are very few instances when you'd turn on a PC, have it either power up or not, and be able to say, "Oh, that's a motherboard problem." Motherboard failures usually show up as second level problems, like "I've replaced the video card and the screen is still dead." If you want to learn how to repair PC's without swapping every part, it's critical to know what to look for on a powered up system, like a CPU fan that isn't running, or RAM that stays cold. Sometimes you can spot a blown capacitor on a motherboard, but it's not a common problem.Edit
Students who are still learning the basics of computer repair, like what components are involved in what operation, will frequently assume that all boot issues are due to a hard drive failure. The truth is, of all the four subsystems represented in this table, hard drives are probably the most reliable. I don't mean that hard drives last longer than memory modules or video cards in the pure MTBF (Mean Time Between Failure) sense, I mean they are rarely at fault when you're called in to repair a PC. Hard drive failures are generally pretty easy to troubleshoot, in part because the operating system will include tools to report on the hard drive's reliability when it's accessible..Edit
I'm putting some thought into whether or not I want to extend this guide to how to troubleshoot basic software issues. The problem is that the number of variations are pretty much without limit. Still, I've seen people go nuts, assuming that they have a virus when it's really a flaky power supply or an overheated CPU.