I wouldn't mess around with old computer power supplies. They require a load to function correctly. For about $20-30 you can find regulated 12VDC power supplies (UL listed) that will provide safe, constant power.
I would also recommend splitting your lighting up and running it on DC. If you use LEDs, you can power a whole bunch of lights off 1 power supply.
Keep your AC power supplies for those accessories that absolutely require it. A good AC accessory transformer is the K-Line PowerChief 120F (if you can find it - they go for $50-80).
George