Dead PC

Starting up, fans et. shutting down after 1 min. Checked PS same thing. Cleaned and applied new therrmal paste. MOBO component or CPU failure. Cannot install new CPU without installing new motherbrd. Sean (Carrigaline) agreed that it has to be MOBO

Desktop not turning on but LED on MOBO glows

Purpose built PC. Plugged in LED light on MOBO comes on. Fan spin for half a second. No image on screen or fans spinning. Tested PS, memory, took out all cards, disconnected HD,CD etc. Took out CPU, applied paste. None of this made any difference. Tested power switch by unplugging from MOBO and shorting (with a screwdriver) across pins. Fan spins -as above for a sec.
So the motherboard has failed it seems.