Dominican1 asked the Answer Line forum why his PC shuts itself off partway through the boot process.
A PC that starts, then fails to boot, probably suffers from an overheated CPU or a bad or underpowered power supply unit (PSU). But I'm going to suggest you check the power source first. Not because it's more likely the culprit (it isn't), but because it's easier to diagnose and fix. If you go to the trouble of replacing the PSU, and then discover that you only needed to plug it in somewhere else, you'll hate yourself in the morning.
So let's get the easy stuff out of the way. Try another outlet on the surge protector. Try another outlet on the wall. Try another surge protector. If the problem is with a desktop PC, try another power cord. If it's with a laptop, remove the battery and try with only the AC adapter.
None of these helped? Okay, I didn't really think they would, but they were worth trying. Now let's get on to the more likely issues:
A serious ventilation problem could cause your CPU to overheat and shutdown before the boot is complete. Make sure the PC's fans are all turning properly. This is easy to do on a desktop: Just open the PC and watch the insides as it boots. It's more difficult on a laptop, but if you listen attentively you should hear the fan.
You should also check the vents and air passageways to make sure they're not blocked or clogged with dust. If they are, use a can of compressed air to clean them out.
I'm tempted to recommend software like the free SpeedFan to monitor your CPU. But if you can't boot your PC, it won't help.
Speaking of software, if the failure happens while Windows is booting, the operating system may be the problem. Try booting into Safe Mode: Boot the PC and press F8 just before the Windows log-on appears (it may take a few attempts to get the timing right). At the Boot Menu (assuming you get that far), select Safe Mode. If you can successfully boot into Safe Mode, it's a software problem. See My PC Won't Boot for recommendations.
If you can't even get into Safe Mode, try booting from something other than the hard drive. You likely have a bootable CD or DVD lying around (a Windows installation disc, for instance). If you don't, try Puppy Linux. It boots quickly from a CD, and gives you access to the files on your hard drive (a real benefit if your PC won't boot). Puppy downloads as an .iso file; double-clicking the file will likely bring up your burner. If it doesn't, download and install ISO Recorder.
If all else fails, try replacing the power supply unit. Desktop PSUs are inexpensive--you can get a good one for less than $40. They're reasonably easy to replace, although if you're not comfortable working inside a PC, go to a professional.
For a laptop, try replacing the AC adapter. You'll need one specific for your model.
Read the original forum discussion.