Try unplugging any and all USB things. Then try it without any HDD or ODD. If that fails it's usually the main board.
my laptop starts itself (no power button is touched) as soon as i put the battery in and boots to a black screen with an underscore in top left hand corner for approx 1 minute and then reboots to the same screen, this also happens with the power cord in. all hardware has been checked ie hard drive, memory, keyboard, wifi card and dvd drive. i do not get any joy from pressing any keys ie f2, f10, del, f12 or anything i cannot even get to safe mode or into the BIOS, any help in advance would be greatly appreciated.