Hi. In my project I need both 5V (for Arduino, relays and lcds) and 12V for some fans. I have an old ATX power supply that I would like to use. I bought the board you can see in the picture. I heard that in some cases you need to short some wires in order to make the ATX work. Is it necessary even with that board? Also, many cables, with different connectors, remain unused (those in the left in the second picture): can I cut them in order to make the whole project less bulky? Thank you very much.
For an Arduino project, use a 12vdc power supply and a 12vdc to 5vdc converter.
The image of the "board you bought" is not a power supply. The picture of a generic power supply does not indicate any of the design. This is a case of "if you must ask, you will not understand."
Here are some fine instructions for that "picture" you have...
You might find that info in the datasheet/schematic/advertisement from the seller.
I know that the board I bought is not a power supply, it's just an interface for the atx power supply that can provide both 5V and 12V. That was not my question 
The image of the "generic power supply" was just to show the amount of unused wires. If you want more details I can provide them for sure.
Many of the old ATX supplies require a minimum load in order to regulate.
I'd check the 5V and 12V outputs with a DMM beore I connect it to an Arduino
PS_ON#, pin 16 on the ATX main connector need to be connected to COM (common) but that may already be done on your adaptor board
Also, many cables, with different connectors, remain unused (those in the left in the second picture): can I cut them in order to make the whole project less bulky?
Sure, just make sure you insulate the wires so that they don't short.
That's the hard drive power supplies and so. You can of course use them as well; probably black for 0V, red for +5V, yellow for +12V. I don't think you will find the -12V (blue?) and +3.3V (orange?) in those wires.
Indeed to switch on an ATX supply some wires have to be shorted; green to black iirc. Whether that's done in the board you show I don't know, you have to ask the seller. I'd expect it to be so as it's a standard ATX thing.
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