Many articles have been written about using power supplies from discarded PC’s, in the shop.
If you enter “using pc power supply for 12v” into the Google search, you will find many, many links.
Links to a few basic articles or documents that I have found are below.  There are many more so do some research.

Inexpensive PC power supplies often radiate a lot of RF noise, rendering the HF bands unusable by a nearby receiver.  This noise can often be eliminated by incorporating a “brute force” AC line filter at the AC input to the computer’s power supply.  Often there is room in the power supply enclosure to add such a filter.

The filter acts as a barrier that only allows the desired frequency of the AC power to pass through while blocking or attenuating the EMI/RFI generated by the PC power supply.

Where would one get a ready-made “brute force” AC line filter?  Discarded microwave ovens include such a filter at the AC line input.  It is usually on a small printed circuit board and can be removed as a unit and installed in the PC.   Discarded microwave ovens should not be hard to find.

Another helpful hint is to snap a couple of RF suppressors onto the computer’s AC line, close to the PC.

G6NGR has provided a document that will help you when using PC power supplies:  Identifying PC Power Supply and Connector Pin-outs 

Convert an ATX power supply into a regular one for your shop

Use That Old PC Power Supply as a High Current +3.3, +5 or +12 Volt Resource

Here’s a Video about using PC Power Supplies

Convert a PC Power Supply into a 12V Battery Charger