Many years ago, I investigated using a computer power supply for a high current project. I found that many of these power supplies only regulate one output. Either the 12V output or the 5V output. And because of this, the more current you pull out of the regulated side, The higher the voltage on the unregulated side. If you use the unregulated side, You have to pull a significant amount of current out of the regulated side as well. This made it very inefficient, and unsuitable for use as a power supply. Instead of trying to make do, and probably spending a lot of money and time trying to salvage a ATX power supply, You can buy one suitable for your application. Such as,
if you're in the US, for $26 you can have a power supply that is both efficient and Safe. if not, you can probably find one for a reasonable price in your country.
I know people have used ATX power supplies successfully, but you need to find one that is regulated on both the 5V and 12V outputs, separately. One that does not require pulling current out of the unused side in order to get voltage out of the needed side.