it could be that you may have done some electrostatic damage. Do you ear wrist straps or ground yourself first??? Also is the HDD spins that means it is good so you could rule that out. Also try the original PSU, yours could be overvolting or undervolting. You should make sure that everything on the motherboard is hooked up correctly. Last your ram is fine because you would at least get some fan action before the comp shuts off so it may be the the power switch lead became loose or is in the wrong spot.