Sometimes your system appears to boot, but doesn't. All the fans will start up and run for a few seconds, and then the system will shut down. Another symptom that's similar, but usually caused by a different issue, is the system starting up and shutting down, repeatedly, with no human intervention. For such cases, here are a few things to try:
The fans run briefly, and the system shuts down. The most common culprit I've discovered is an improperly seated CPU cooling fan (or a disconnected or broken CPU cooling fan power wire.) If the CPU cooler isn't properly seated, it will overheat incredibly quickly. The system then shuts down to protect the CPU.
The system enters a power-up/power-down repeating cycle. This can be a sign that the motherboard is defective. If it's not the motherboard, then make sure the CPU and memory are properly seated. I've found that the problem occurs most often when I'm trying to use an old power supply in a new system. Newer motherboards with current-generation graphics cards sometimes require higher startup current than some older PSUs can deliver. The result: the system keeps trying to start, but doesn't get quite enough juice.
Diagnostic LEDs and PC speakers can be your friend here. Most new boards have diagnostic LEDs. The pattern and color of these can help narrow down the possible sources of the problem.
Some systems even have alphanumeric LEDs. Unfortunately, these are often poorly documented in the motherboard manuals, though you might be able to find them listed on the manufacturer's Website. Sometimes, though, you have to resort to Google to find out what the codes mean.
Is your PC beeping? This might be a clue--the beeps are generated by the BIOS as it detects errors. For example, one short beep means all the POST diagnostics have passed. Two short beeps usually means the system has found a problem with memory. One long beep, followed by two short beeps often indicates a problem with the graphics card. Here's a complete list of diagnostic codes.
Many modern cases no longer ship with small case-mounted speakers. Some motherboards (mostly Intel-designed boards) have onboard piezoelectric beepers. If your case lacks a speaker, and your motherboard doesn't have a built-in beeper, you can find tiny beepers in shops and online that plug into the speaker connector of the motherboard.
You can see video, but the system hangs before boot. Sometimes, everything seems great--until the system hangs during POST. You can see either the motherboard maker's logo screen, or some text. The three most common places I've seen these boot locks is just when text starts to appear, at the storage controller enumeration, or when the USB controller is being checked.
If your system locks up at the first sign of text, your memory might be incompatible or faulty. Try some spare memory modules if you have any lying around.
If the system hangs while the onscreen messages say it's checking USB, try a different keyboard or mouse. While it's not supposed to happen, I've had expensive keyboards turn out to be incompatible with certain combinations of chipsets and BIOSs. Often, updating the BIOS can cure these issues. Also, make sure no other USB devices are attached. On one occasion, a USB flash memory card reader had gone bad, and having it plugged into the system caused the PC to hang at the USB section of the POST.
I Can't Boot!
Boot problems are pretty common with new builds, but fortunately the fixes are often simple. Let's look at some of the more common headaches.
Can't find the boot disk? When this happens, you'll see a message that indicates an unformatted disk, or the operating system can't be found (with an unhelpful message asking you to restart). In a new system, this often means you've got the wrong boot device specified in the system BIOS. For example, if you need to boot from the optical drive to install the OS, you need to set the optical drive as the first boot device. It's possible to be fooled by this boot-device consideration.
Some newer boards with both IDE connectors and SATA connectors will treat them differently. You may see "CD-ROM" as the boot device, set that, and the PC will still fail to boot. You check the BIOS only to find a different device listed as "ATAPI Optical Drive" or something similar--which turns out to be the correct device. Similarly, if you've got multiple hard drives, make sure the hard drive order is set correctly, because specifying "hard drive" as the boot device means only that the system will attempt to boot from the first hard drive.
Blue screen on startup. If you're trying to boot from an existing Windows installation on a hard drive transferred from an older system, then a BSOD on startup can occur. This often means that the system can't find the right storage controller; the actual error code is 0x0000007. Maybe your old system had its SATA ports set to IDE mode and your new one is set up for AHCI. Or perhaps your new board has a different chipset.
If this is the case, you may have to resort to a Windows repair installation, which you can perform with either Windows XP or Windows 7. The repair install option is available when booting from the Windows XP CD if you're running XP. (If you're running Windows Vista, you may have to reinstall from scratch.) With Windows 7, you have two options. One is to boot from the Windows Setup DVD, and select the Repair my system option. The other is to press F8 during the boot process to get the Windows boot menu and select Repair your computer.
Can't install Windows! So you're happily installing Windows, and the setup process aborts for some reason. It may be a blue screen. It may just hang. It may stop and tell you that it can't continue.
One of the most common culprits here is either bad or overclocked memory. The Windows boot CD ships with a memory diagnostic. Just boot from the DVD and run the diagnostic, which should tell you if memory is the culprit. Boot from the Windows 7 setup DVD, pressing F8 to get the boot menu. You'll be prompted to either load Windows (which is Windows setup) or the memory diagnostic. Alternatively, you can schedule it from within Windows. In the Start menu search box, type mdsched and press Enter. You'll be prompted to either restart immediately to run the diagnostic, or to schedule it the next time the system boots up.
Windows Setup is also extremely sensitive to overclocking. Even modestly overclocked systems, which might be stable when running an application, will often crash when running Windows setup. So always, always install Windows with your system set at standard default settings. Save the overclocking until after you've installed all your hardware and drivers.
Wash your Windows setup disc. If there are visible fingerprints, cleaning the disc is fairly obvious. However, this often works with brand-new discs. Sometimes oils or residue from manufacturing can make a disc unreliable, even if your naked eye can't see the problem.