Hola,
if you browse the Forum, you will come across many who thought just like you, that becaus the Fans spin that the PSU is functioning... this is just not true...
one of the Rails, may not be delivering enough Power, or not function at all... could well be the 3v line, or the 5 volt line... so testing with another known working PSU, will tell you if it is the PSU or not, just to put that out of the equation...
also, I suggest unplugging the IDE Cable from the MOBO side, and the USB front header (if attached), check the seating of the GFX card, as just a nudge off, can also cause the symptoms you describe... check the seating of the HS-fan for orientation... 180 degree offset on AMD's have been known to be a culprit aswell...
another thing I would do, is to clear CMOS... some boards do not like to boot if this is not done at the beginning...
Ben
"If it works don't fix it! If it doesn't use a sledgehammer..."