So I have this led light strip thing which is 12 volts, so is the Molex 4 pin connector. So I found out which is the plus/minus on the wires and the Molex. When I connect the two, the led goes on. When I leave it connected for more than a second, the Molex gives off a spark, and the comp restarts.
Yes, but you have no idea how much current it is drawing. A car battery can supply an extra 10 A easily, an extra 10 A load on your power supply would be quite significant, and if its a cooler master extreme power plus 500 W, 10 A extra on the 12 V rail would throw it over its limit.
I see no mention of how much power those lights need, which means you have no idea how much current its trying to pull from your power supply.