OK we'll go from the beginning.
Disk Boot failure basically means that the BIOS can't see a boot disk.
Not it's not there, mind you, but it just can't see it.
Couple of things:
a. Get into your BIOS. Where it says Primary Master, it should show your drive number Like WD456789 or something like that. If it does not, press 'Enter' and a seek sequence should start. Your hard dive should be detected at the end, with all sector info and stuff.
If it is not, check:
1. the ide controller your hard drive is on, it should be on the lower numbered one. Not the lower one, just the number: usually 1 or 0, depending on the manufacturer of the board, who follow schemes like IDE0/1 and IDE1/2.
2. The jumper on your hard drive, should be set to Master ideally but should boot OK if it is on slave. Master is preferable. Since you were OK on the last system I doubt it'd require tweaking.
b. Again in BIOS: Check your boot sequence. Should be Floppy, CDROM, and IDE-0 in that order. you may want to disable the first two and have the system off of IDE0 in the first place. I like to keep it on because, well, just because. IDE 1,2,3 refer to the other IDE connections. Not required except if you're booting off another drive.
Your error message probably means the BIOS may not be looking for a hard drive at all. The Asus BIOSes need to be explicitly told where the hard drives are. I actually like that system as I can't stand the BIOS looking for the drives each time on bootup.
c. In any case since you've changed your mobo I would strongly advise a fresh format and install. You'll have a lot of junk floating about if you don't do this, and it's a little dicey anyway. A format shouldn't take you much time.
d. Since it's been a while since I looked at an Asus BIOS, I'm not sure where to look for this, but it must be under that section. You should press Enter under that option (IDE Hard Drive) to get to the screen you want.
e. Are you sure you got the cables in correctly? Double- and triple- check them. Forget the LED indicators for the time being, concentrate on getting your system working. When you do, read the manual to figure the connections out.