I'd measure the voltage first as many ATX supplies need a minimum amount of current to regulate properly, so with no current load (or the relatively small current load of an Arduino) you may see 12V turn into 13V, 14V, etc.
But if it's steady at 12V, you should be OK.
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The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected