Computer won't boot?
Determining the cause here is half the battle. Is it hardware- or software-related?
Your PC problems solved
First, check all cables to ensure everything is hooked up tight – including the electric socket. Does the power supply turn on? Listen for its fan or your hard drive spinning. If you hear nothing, your power supply probably needs to be replaced. Test the voltage output with a power-supply tester. Check whether the socket is at fault, too, by plugging in another device.
If your power supply is okay but nothing appears onscreen, plug in a different monitor to ensure it's not your display that's blown. If the replacement monitor works, try replacing the video cable. If that doesn't work and your hard drive spins normally, your graphics card is probably at fault.
Replace it and, while you're inside the case, ensure that all the fans work at startup. You may have excess heat issues.
If your monitor is working but you detect no hard drive activity and see no display – or you see a display but the PC can't get through bootup – reset the CMOS. Shut down the PC, unplug it, ground yourself and take out the battery on the motherboard. Wait five minutes before resetting the CMOS jumpers as advised on the manufacturers' website. Now reboot your PC.
If the PC still isn't functioning, bad RAM could be the culprit. Remove one memory module at a time – or replace each module with a known good one – and reboot after each test. Alternatively, create a free MemTest86 boot disk on another PC (memtest86.com) and try using this to test the RAM.
If none of this works, your motherboard or CPU (central processing unit) is probably damaged and needs replacing. However, your data can probably be recovered by installing your hard drive on another PC. Consider getting an estimate from a repair shop, but it may be more cost-effective to replace the PC. A repair shop might be your best (and only) option if your PC is a laptop.
Finally, if the PC's Bios routine runs but the drive won't spin, your drive may have crashed.
As dire as these hardware failures can seem, you're far more likely to encounter software issues, such as Windows refusing to start or freezing while it's loading.
If your OS is the problem, boot into Safe mode. As Windows starts up, press the key as directed to reach the boot menu and enter 'Safe mode'. Often, Windows will recover if you boot into Safe mode, then shut down and reboot normally.
With Vista, select the 'Repair Your Computer' option at the boot menu. You'll find choices to aid your PC.
No joy? Try 'Last Known Good Configuration' at the boot menu. This is helpful if you've changed hardware or drivers. Remove new hardware – it may be incompatible – and roll back drivers in the Device Manager. Right-click My Computer (Computer in Vista), click Hardware and choose Device Manager.
If you can run in Safe mode but can't start the full version of Windows, try System Restore – Programs, Accessories, System Tools in XP; Start, type system, System Restore in Vista – to roll back your PC to a happier time. Run an antivirus and antispyware app in Safe mode, too.
If you still can't boot, you probably have heavy-duty Windows problems. Try to boot from an emergency CD using Knoppix (knoppix.com) or Active Boot Disk (ntfs.com/boot-disk.htm), which can help you to see whether your PC will boot at all and to collect any critical files from the drive.
If your PC is still unstable, reinstalling Windows is probably your best bet.
Avoiding hard disk issues
PCs die unexpectedly, so focus on 'what if'. Turn on System Restore, keep your system recovery discs handy, back up often and keep a spare hard drive and power supply.