1. )I have asus p5pl2 motherboard and i had the same problem.
Solution 1: For me there was the little tiny wire in a wrong place !
2. )And later i had another problem that when i turn my computer on then it turns on but it keeps restarting and turning on !
Solution 2: I needed another and better power supply ! (I had 400 watts ; I needed 750 watts , min:600 watts!
I hope im helpful ! Thanks !