When your PC won't boot

There's nothing to make the blood run cold in your veins like a PC that won't boot - and Murphy's Law ensures that it happens at the most critical time. It happened to me a couple of weeks ago. Fortunately, I had done a major back-up of data a short time previously, so was not rendered a total gibbering wreck on the spot. When I got the machine back from the repair shop, it turned out that a cable from the DVD player was shorting on the case - not something I would immediately have thought of. A simple problem, but it did set me back $90 for the tech's time.
Before you run to the shop, there are quite a few things you can do yourself if your PC won't start. This PC World article by Lincoln Spector gives you a good run-down in simple language. My only comment is that I am surprised he does not recommend pressing the F8 key on start-up (to start the computer in Safe Mode) as a first restort. In the past I have found that effective in at least 50% of cases.