If its a ATX power supply, it dont work without motherboard. You must make a bridge to do it work. It inst difficult, but no i dont remember exactly how to do it.

In the other hand, use a 250w (or 400w) power supply to run fans with low power is a waste, and with so little load, the supply voltage can be quite inestable. It would be a great idea to use it to power all the setup (using 12V CFL and leds, and the fans).