I wouldn't put a (possibly) bad PSU in a good machine--you may wind up with two dead computers. The easiest way to test the "bad" PSU is to jump the green wire on the ATX connector to any of the black wires. This "turns on" the PSU. Then, test the output, 12v, 5v, 3.3v with a multimeter.
PSUs do go bad sooner than expected sometimes. I had a Powerstream 520 go bad after a few months of use earlier this year. While it was being RMA'd I bought another (1 day shipping from Newegg) as a replacement. The new one showed up DOA. So, I ended up RMAing two of them. OCZ was very good about replacing both units, and the system I'm typing on is being powered by one of the replacements (I sold the spare in the classifieds and as far as I know it's still working fine as well).