Based on what little I can see in the photo, that looks like an ATX power connector on an NLX form-factor riser board.

NLX wasn't really a generic DIY PC option, so I assume you're looking at some PC built by some well known PC builder with a particular model name on the front, or on a label on the back. Knowing what brand and model PC this is would help to provide a more concrete answer.

Yes, beware of Dell and the like switching about 12V pins on their power connectors so that it will blow up when you connect it to a generic psu.