I'm trying to put together my first build, and I have a big issue. When I plug in the 24-pin ATX cable, everything runs fine. The case fans, cpu fan, graphics card fans, hdd, and dvd drive alll work fine, and obviously the CPU doesn't turn on. However, once I plug in the 8-pin 12v ATX cable, everything goes wrong. The CPU fan spins a bit then stops, and the internal speaker gives the repeating short beeps that mean power failure. What am I doing wrong?