Power supply, motherboard and CPU. I'd start by replacing the cheapest part first, then the next and then the last part. If it still fails then you still have one to three failing parts.
I am working on a Dell Dimension B110 and got the blinking orange light. No system fans turn on, but the led on the motherboard ligts up. After some troubleshooting, I hooked the 4 pin 12 volt cpu power cable to my power supply tester. MAGIC, The system fan came on, and the orange light went solid like there was a component missing. It makes me think the cpu is bad.