i would suggest a different power supply as well.
one can usually be found for between 30 and 40 bucks.
unless you go way budget or high end...
and what light are you referring to?
some motherboards have an led on the motherboard itself, but i am not sure you arent talking about the light on the tower.
when you have it unplugged, then plug it in, and hit the button, does the light come on?
if so, are the fans/hard disk spinning?
if not, do they even twitch?
does it make any odd noises?
will the cd-roms open if you try to open them?
these are all the things i know would be helpful.
chances are, the supply is dead, or there is a peice of hardware thats halfway plugged in. (or fried)
try a new power supply before you rip into it too far though.
after that, you might try speedo's fail safe plan, and try to start it with the mobo laid out, only cpu/fan, and ram, and video, and see what it does.
then one by one, put stuff back and see what stops it.
ive seen a fried pci/agp card stop a machine from booting, as well as bad cpu.