Well if it seems to have life but then dies again i would be tempted to say the psu is ok, if you had just built the pc i wud say it was shorting, but what i think could have happened is u underpowered the pc, by adding another hdd the power supply was stretched too thinly and may as a consequence damaged the board or cpu...a friend of mine did exactly the same, he ended up replacing board, psu and cpu.
Definately worth getting hold of a new psu to try if it doesnt help try each of the components in turn to establish which one (if not more) have gone
