LightBlue25,
When you power-ON the motherboard, do you get any BEEPS from the motherboard's onboard speaker? If so, these are POST errors which are indicating a problem with the motherboard, memory, processor, etc.
Are you able to boot into the motherboard's BIOS SETUP? If so, there should be a screen in there for monitoring the CPU/MB Temps as well as the fan speeds and possibly the voltages.
Check out:
http://dlsvr01.asus.com/pub/ASUS/mb/...737_p5l-mx.pdf
If everything was working prior to your re-build of the system, the something obviously changed.
If the problem occurred PRIOR to this, the either your Power Supply or Motherboard have likely gone bad.
To eliminate the possibilities of shorts or faulty drives or cards, disconnect ALL hard drives, CD/DVD ROM drives, Add-on cards (except for video) from the motherboard. Power the system on with THIS configuration and see if you have any luck.
If the system comes up this way, then something is faulty with one of the devices that you removed.
If the system still fails to boot with everything but the video and PSU connected to the motherboard - then the problem is with either the PSU or Motherboard.
Trying to help here, but need more information.
- John