why my computer suddenly shut down even i booting up it ?

And from your post eyeing not able to understand whether your computer restarts before the operating system starts to load or is it after the operating system has floated. If your computer is restarting even before the windows started and I think there is something missing with your hardware. Check that your computer is not getting overheated if your processor is not fixed properly and sometimes eats up very quickly and so it automatically restarts. If the windows as loading and definitely there is no doubt that there is some problem with your routing system and I would suggest to go through safe mode and then load your computer operating system find out the problem with the programmes by going to a mess config through the run, and check them out. It still the problem persists and I think we have to format your computer and install the operating system again.

Most of this problems occur when the CPU fan has failed or the it has shifted from its original are and has a loose screw that keep it out of position. also if you have checked that and still it restarts then you might consider reinstalling windows, there might be some missing system files. sometimes is very difficult to tell if you are experiencing a hardware or a software problem because the system do not report in cases like yours where i computer turn itself off. But i suspect the fan, do check it and if is well seated then reinstall windows

Having a computer which suddenly shut down while booting up means it is not possible for you to enter Safe Mode. The suggestions given below are based on my personal experience. This is in addition to the suggestions in other responses. 1) Unplug your hard disk and reboot. - If there is no problem in the rebooting process where you would get a command prompt then your problem may be related to the hard disk. Try again with the hard disk plugged in. - If there is a problem most probably it is caused by the RAM module. If your computer is old then you may have to replace the whole unit. 2) Windows may need to be reinstalled. As more and more files are written on the hard disk there would bound to be errors. rosdimy

Maybe you should have your power supply checked? How up to date is your antivirus and spyware?