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A number of things can cause a desktop or laptop computer to constantly reboot without ever actually completing the Windows boot process. The causes can range from a buggy patch or device driver to a hardware problem.
When dealing with an infinite reboot loop, the first thing that I recommend is to try to boot the computer into Safe Mode. You should be able to do this by pressing the F8 key during the early phases of the boot process. Safe Mode offers very limited functionality, but it can help you to diagnose and repair the problem.
If the computer boots into Safe Mode, then you can usually (but not always) rule out a hardware problem. When a computer boots into Safe Mode successfully but will not boot normally, the problem is almost always related to a bad device driver or to some other piece of software that is loading during the Windows boot process.
Disable everything nonessential and reboot the computer. If the system boots normally, then try re-enabling drivers and software, but do one thing at a time, rebooting between each. This will allow you to tell which component is causing the problem.
If booting to Safe Mode does not work or if you are able to boot to it but the system eventually reboots anyway, then you may have a hardware problem. If the computer is running a version of Windows that is earlier than Windows 8, then look for the infamous blue screen of death (BSOD). This screen contains error codes that you can use to help diagnose the problem.
If you do see the BSOD -- or the Windows 8 equivalent -- the problem is very often related to bad memory. Swapping the memory out may cause the problem to go away. If that doesn't work, then you might consider reloading Windows and testing other hardware components.
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