To start a Barebones test, have only the motherboard, vga, psu, ram (1 stick) and proc set up and see what happens upon boot. then add and subtract parts from there after analyzing the results until you've found the culprit. There can be a lot of trial and error involved. I believe it's usually recommended to remove the motherboard from the computer case to prevent a short while doing this. Asus' site has an okay pictorial guide:
ASUSTeK Computer Inc.-Support-
Click on "Motherbaord" and "Trouble shooting PC"