You should have already done this before trying to power up the PC, but a lot of novice PC builders will skip this step in anticipation of powering up their new machine. This step may save you time and money. First, triple check to make sure the CPU is sitting flat on the motherboard socket. It's probably hard to tell because of the heat sink, but if the entire CPU and heat sink combo looks skewed, then chances are it's not installed properly. Next, go over the IDE cables and the thin wires running from the power and reset switches to the motherboard. While the IDE cables are keyed, it's quite possible to get those case wires backwards or completely. Lastly give the PC a little shake. Make sure an errant screw didn't fall behind the motherboard during the install. This could blow the motherboard or cause the system to go to ground.
