Sounds like the machine is esperiencing a fatal system error and is shutting down to protect itself. My cut at this would be to undo whatever it is that you did that caused this problem to start. If you installed something and then it immediatly began to screw up then that is most likely the cause. If you can get it to begin the boot process then you should be able to go into safe mode. Dont know for sure but I think that you hit the F8 key at some point during bootup. If you can get into safe mode then uninstall what ever you installed last.
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