Pull the CPU, Reseat it, Clean the surface of the cpu of old thermal paste, put on new thermal paste and make sure the heat sinc fan is turning. Plug in one stick of memory and the video card and then attempt to boot. Pay careful attention to the LED lights on the keyboard....do they blink during the rapid boot of the bios (POST)? Does the CPU fan spin? Don't just assume it's the CPU. It could be the power supply. If the ATX connector voltage is low enough the bios will read the error as the CPU and you'll have a dead system issue to trouble shoot. I have seen people rebuild entire systems because they didn't do the basic and simple trouble shooting for a "dead" system. Tell me what happens when you attempt the CPU/video card/memory boot.
Edit: What mobo model and manufacturer? We need to know the Bios (AMI, Award, Phoenix etc. The codes are different for each.)