...but if this is an old AT board, it should just power up. no soft start here.
take a volt meter and double check you are getting 3.3v, 5v, and 12v on the orange/red/yellow wires (respectively) if the board is getting power, it should startup (barring a catastrophic failure in the past)
if the old power supply was no good and failed in a most spectacular way, it may have taken out the regulators on the motherboard, in which case, it may be junk.
double check you are indeed getting juice out there first though.