Yeah, we need some more info.
Generally putting an older PSU in a new computer does not cause any problems providing the wattage is high enough for the devices you are running.
As CarnageX said, give use the specs of the PC and all the info you can find on the side of the new + older PSU and we'll be happy to help you out.
It depends on what you are replacing it with, and what the orginal one was - as said before it's not the watt's that matter it's the amp's on the 12 vault rail.