Is it possible to design a 4-pin to 24-pin atx molex converter?
If you want to power a pc from a regulated 12 Volts, you should look into a compact SMPS setup. Or you know, just use a commercial product like a Pico PSU. Completely depends on how much power you need.



As far as using Zener Diodes, based on typical ATX current needs, you will need a huge Zener or heatsinking. It's not even close to ideal. Using a Zener Regulator calculator, a 500mA load at 12V in, 5V Out, will require the Zener to drop 2.55 Watts in heat.

Linear regulators would be much better. Switching regulators the best. 

For -12 Volt, you will need a voltage inverter. But -12V is a legacy voltage on ATX supplies for normal PCI cards. Not really used. It depends on what you have connected. Many times it can be skipped on modern motherboards.