Okay do it like this. Remove all the hardware parts other than Processor, Mother Board, RAM, VGA and Keyboard.
Remove all HDDs, CDROMs, FDDs, USB devices, mouse, soundcards, modems, printers etcc..
Now boot your machine and see if it still restarts. It should boot up to O/S loading stage and stop saying no boot device found or with similar message.
If it still restarts then you have some hardware problem. If that happens let me know then I’ll tell you how to do that.
Well if it never restarts then turn off the machine and plug the hard drive. Then boot it again. Then if it restarts it most probably your OS having problems or your Hard drive has problems.
Anyway clear CMOS will help too. If you know the CMOS jumper you can reset it. If not turn off the machine and remove all the plugs to wall outlets. Then remove CMOS battery and press the power button. Leave it like that for 1minute and put the battery back in, plug power and boot the machine.