Here is the link Mike M. posted a few weeks ago
http://wiki.ehow.com/Convert-a-Computer-AT...ab-Power-Supply OR to make one like I have been making, I can tell you pretty easy I think.
First if it an AT type power supply with an external power switch then just rig up the switch how ever you like but if it is an ATX type, which is the newer type that gets it's on/off signal from the motherboard then you will need to short out two wires so that it will power on.
(this part for ATX only)
To do this first make a short piece of wire that can be used to short two of the wires in the main plug that normally plugs into the motherboard. Then you can try and error to find which two will make the power supply turn on, it will not hurt anything if you short the wrong 2 leads because the poser supply will not turn on unless you get the correct two. Just be sure to only short one pair at a time. Now to get you close so you don't have to try every combination, it is the 3rd wire on one side to the 4th on the other side, every one that I have done so far it is a green wire and a gray wire. So if you look at the plug then the 3rd from one end on one side and the 4th from the other end on the other side. If you locate those wires and short them and then turn on the switch mounted on the back next to the connector for the cord, then it should turn on and the fan should spin. If it does then you can cut of the plug and cut the rest of wires from that plug off as close to the locig board as you can being sure none of them can short to the case, the board or each other.
(this part for AT or ATX)
The rest of the connectors that are normally used to supply power to the drives in the PC each have several wires, red, yellow and Black. The yellow wires are 12V positive, the red are 5V and the black are ground. Take one Yellow and the closest black wire and cut them off the plug then attach them to female bannana plugs and mount the plugs to the case of the power supply. I always test the pair I want to use with a volt meter to be sure it is 12V. You can use more than one of the yellow and black pairs and mount two or more 12V pairs if you want. Then again carefully cut the rest of the wires short taking care not to leave anything exposed that can short. You can make a 5V output also using the red and black pairs if you have a need for that but I don't know what that would be related to RC stuff.
Also when you mount the bannana connectors to the case be sure the conductive metal on the plug does not short to the metal case of the power supply.
LAst note on the subject, some power supplys have higher output AMPS than others, most have a label on the side. I have made them with as little as 2 AMP on the 12V output and as many as 10 AMP. Obviously if you are running a charger that can charge multiple batterys you will need as many AMPS as you are trying to charge. In other words if you are charging 2 batterys at 1.5 AMPS each you need at least 3 AMPS output on the 12V (probably a little more for whatever power the charger uses itself). The ATX's usually have higher AMPs.
IF you bring the power supply to the field I will show you if you have any more questions.