I just put my old computer back together. It won't turn on. I've spent the last hour troubleshooting it. I've triple checked all the wires. It's the exact same pieces (other than a CD drive but I unplugged that incase it was the problem that didn't work) that were in it before when it worked. I've swapped the hard drive incase it was the problem. And that didn't work. The light on the motherboard is on so I know the computer is getting power.

Specs:

400W power supply
ASUS P4B motherboard
P4 1.6 Ghz no HT CPU
256MB PC 133 RAM
Geforce FX 5600XT 128MB
80 GB IDE hard drive (or a 10 GB for the other one I tried)
Windows XP Home Edition SP2 (on both hard drives)

EDIT: I fixed the problem. Turns out I had two of my jumpers in the wrong spots.