Check the mobo for bad capacitors, bulged out or leaking capacitors can cause an issue such as yours. Try removing hardware devices that are connected to the computer (unplug everything except for the power cord, monitor signal cable, keyboard and mouse). If it still keeps rebooting try a known good keyboard and mouse from another system. Still no changes in symptoms? then try removing any pci cards, disconnect the optical drives, .... then test. Still have issue disconect the hdd as well. If it boots to the post screen without shutting down, then leave it on the error screen that says no operating system found. See if it will reboot. If it is still rebooting then swap memory with known good memory from another system. Try reseating the power cables to the mobo. If it still happens after doing all these things and you swapped the system with a new powersupply then you know you either have a problem with the CPU or the mobo.
