There are so many components inside a PC that it can sometimes be difficult to determine which one is causing a problem. If your PC won’t start or is crashing regularly, the problem could be a hardware fault.
The best way to determine if you have a hardware fault is to perform a minimal hardware boot. To do this, first remove all external devices from your PC except for the mouse and keyboard. This includes USB-attached devices.
If your PC is still malfunctioning after you disconnect all of the external hardware, then open the case and remove the following components.
• The optical drive
• Any hard disks except the one on which Windows 7 is installed
• All but one of the memory cards
• Any expansion cards except the graphics card (if present)
Removing these components enables your PC to start with the minimum number of hardware components. If the computer is still malfunctioning, you can conclude that no external devices are causing the problem. However, if the PC works properly, you can begin adding the components back one at a time, restarting and testing the PC every time you reattach a component. This process of elimination can help you diagnose what hardware component is at fault.
If you perform a minimal boot and the computer still doesn’t work, your task is harder because you cannot remove anything else except the graphics card, and that only if your motherboard has on-board graphics. In this case, the first thing to do is try each memory card in turn, turning off the computer to change the card and restarting it each time to see if the computer works. This will establish whether you have a faulty memory card.
If the problem persists, then it can only be caused by one of four components: the power supply, the primary hard disk, the motherboard, or the processor. I’ll cover how to jumpstart your PC in the next section, which can help determine if you have a faulty power supply, but how do you check the other components?
You can check the hard disk by plugging it into another computer as an extra drive and seeing if any problems are transferred with it. Don’t try to use it as the boot drive on this second PC, because the installed copy of Windows 7 won’t have compatible hardware for the new computer and Windows will fail to start.
If you determine that the hard disk is not the problem, then the processor or the motherboard might be the culprit. Because you can’t start the PC without either of these, you might consider consulting a technical professional. But before you do so, there are some other things you can try first.
Sometimes changing graphics cards can result in a blank display if the currently set resolution is higher than your screen can support. When you are changing a graphics card or removing one for testing purposes to use the motherboard graphics chip instead, it’s a good idea to first lower your screen resolution to 800 x 600 or 1024 x 768. You can do this by right-clicking on any blank area of the Windows 7 desktop and selecting Screen Resolution from the options that appear.
Source of Information : Microsoft Press - Troubleshooting Windows 7 Inside Out