My guess is it's the PSU. Pre-built computers companies tend to cheap out on the PSU more than anything else. But before we do anything else the first thing to do is unplug everything. You don't want to get electrocuted or start a fire if something is burning. Next take a look at the motherboard. Look for any burn marks or bulging capacitors. If all that looks ok I would replace the PSU with a known working one and see what happens. Like I said I really think it's the PSU not the motherboard.