In a situation as this, I might try to boot the computer with only the power supply, motherboard, CPU/HSF and the speaker. If you get no beep, one or more of these are bad.
I have a Compaq Presario desktop that won't start-up. When you push the power button the lights come on and the monitor stays in the sleep mode. I was told that the motherboard or processor were bad. So, I purchased a new Abit VA-10 motherboard and Athlon XP processor. I installed both items, and the computer still has the same problem. Please help!! What else could be wrong with the computer?