Many cheaply-made motherboards (which is not to say that they weren't expensive to buy) come through our office with exploded power supply filter capacitors. Open up the case and look around the ATX power connector region for blown-up or "bloated" caps. Good news is, these usually don't damage the rest of the hardare.

On the other hand, even when they blow up, they usually aren't accompanied by a burning smell, so maybe this isn't it.