strip out the tower and check things one at a time.

This involves Pulling the Power Supply and Pulling the motherboard. Remove everything from the motherboard and place the bare board and Power Supply on a piece of styrofoam or cardboard. With the power supply on the table connected to the motherboard and Power supply plugged in, Use a small tool to momentarily short the two power on pins. The Power supply should turn on and stay on noted by the supply's fan running.

Power off. Now put the CPU/HSF on, add one stick of RAM, and mount the motherboard back into the case.

Connect the Power Switch wireing, no mouse, keyboard or monitor and again see if the power supply turns on and stays on. Get past this point power off and connect the keyboard, mouse and monitor, If needed Get into the BIOS to set boot options. Add one thing at a time checking each item. First drive to add should be your CD. drive then hard drive.