I agree about never buying a pre-built. When you open up the case is the led power light blinking? I've been reading some HP support docs and found a pre-video graphics error. The possible fix is to disable the FDO jumper after resetting cmos. I can't find a straight answer as to what FDO is exactly but it appears to allow you to program the bios firmware and it also may disable ME settings which stands for Management Engine. I know this all sounds screwy but it might be worth a shot.
Disconnect power cord, reset cmos, find the FDO jumper(it's outlined on your motherboard link), place the jumper to the disabled position. Replace power cord and start up.