Pc boot cycleing

I have a dell dimention 2400 WindowsXP and when I turn it on, it stays on for about 4seconds and shuts of and stays off for about 4 seconds then stays on, but it doesn't even trigger the monitor to auto turn on like it always did. But also there are about 4 little lights on the back.. those are A, B, C, D. (and maybe E) They are all supposed to be green but about 2 of them are orange.. HELP!!!
 
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