Here are the instructions I used for building my power supply:

http://web2.murraystate.edu/andy.bat...owersupply.htm

It's easier than it looks, especially if you only need 12V. I had an odd Dell PS that none of the colors matched, but I was able to sort it out with a voltmeter.

All you really need to buy is the resistor (2 for $1.85) and the banana jacks (about $1). The switch and the LED are not really necessary.