Needless to say, something had just broken terribly. My initial suspicion was the PSU, which I started removing from the case. Because it was the middle of the night, however, and my girlfriend was sleeping, I didn't want to continue with the racket, so I let it rest for this this morning. When I woke up this morning, I proceeded to remove the PSU. As luck would have it, though, I can't remove the PSU without removing the motherboard, and this would prove to be serendipitous, as it turned out that it was not in fact my PSU that had broken, but rather a capacitor on my motherboard. I found scorch marks on my motherboard plate, and the burned underside of a component with some copper sticking out. This was clearly the fault, and also the flash I had seen when my case was open.
I had been lamenting the potential (if outside warranty) cost of a new PSU, but this was even worse! Anyway, I take it to the store, hoping that they'll say something like "Ohh, this is a warranty issue, we'll just take your card and give you a new one." Unfortunately, as you might have guessed already, that was not the case. It sounded more like this "Ohh, I don't think this is a warranty issue. I'll have to check with ASUS, in any case it'll take at least 3 weeks". How this can now be a warranty issue is beyond me, but warranty or not, the "at least 3 weeks" easily turns in to "more like 2 months, if you're lucky", I know from previous warranty issues. The concept of being without a working computer for that long was not something I liked, so in the end I bought a brand new motherboard. In my best case scenario I get a new motherboard from asus which I can resell for some money. In the worst case I'd have to buy another one anyway, in which case I'd be out of the money anyway, AND have had to wait several weeks. Hopefully I'll get a new one form them, but you never know. I'd like to think that burning capacitors might be covered by warranty, but you never know.
There's no snazzy end to this. I realize, as I'm writing, that there's no way to sow this bag up neatly, so I'll just leave you with this: fuck...