I just built a new comp. and when i tried to turn the power on it won't detect the monitor and i don't hear the POST beep either. I'm sure I installed everything correctly so I don't know where my problem lies. The fans and such turn on and the LED light for the power is on so I don't think it could be my power supply, which is 400w. If anyone could help me eliminate possible causes I would appreciate it. BTW, when I have the CD drive connected, the light is always on, anyone have an idea about this also? Thanks in advance.
What I'm having trouble with:

CPU:
Athlon XP 2600+ FSB 333
Mobo:
FIC AU13 with nForce 2
RAM:
512 DDR PC2700
Video:
ATI Radeon 9000 PRO 128 MB AGP
Storage:
WD 80 GB IDE 7200 rpm
Audio:
onboard Realtek '97

Take out the ram and re-seat it carefully, then do the same with the video card. Make sure you fit them securely, but not too tighly. Once you have done that, try again. If you still are dead in the water, remove everything except the video card and one stick of ram (if you have more then one) and try again.

I tried using a different stick of RAM and same thing happens. I also tried a PCI video card and nothing happens. I tried running it with just video card and RAM already too.

If your cdrom light is always on, then that is usually a sign that your ide devices (hard drives, cdrom drives etc.) are not properly connected, or have incorrect jumper settings.

Ok, that would isolate it to most likely one of these:

* Motherboard
* CPU
* PSU

I doubt it is the PSU. If you have another motherboard or another processor to test with, that would help isolate it further. Try checking the manual for cpu settings to see if there are any jumpers on the board you have to physically change. Also check the jumper that resets CMOS data; on many boards if this is left closed the board will not boot.

No beeps mean the board isn't doing anything at all, which can mean defective cpu or defective board, among others.

Because you stated the CDROM led always remains lit could possibly mean a bad IDE controller or something else on the board - Make sure you are disconnecting the IDE cables and the FDD cable.

Give those a try and let us know.

also check the soft power on connections,
if your agp card is the wrong type often the motherboard will not start

He said he tried booting with a PCI video card.

Thanks everyone for the replies. I was thinking about trying the processor and mother board out but i don't wanna go through the hassle of using thermal paste each time and all that junk, so I was wonder if its safe you just test boot with the heatsink on the CPU. Just a quick boot then shutdown test if it runs, so I was wondering if the CPU could get damaged that fast if I do that.

Well I figured out what the problem was. I was supposed to plug the 12v thing into the motherboard. The manual told me it was only used for Pentium 4s. Anyways thanks for the help everyone I appreciate it.