Hardware failure: Need to RMA my video card.
Well, it seems my graphics card (which has been limping along for years with intermittent glitches under load) has finally given up the ghost and my computer will no longer boot. Fortunately, this occurred after I finished wringing many hours of enjoyment and completionism from Deus Ex: Human Revolution, or else I would feel incredibly annoyed at the interruption. (Aside: It’s a good game. A worthy successor to Deus Ex.)
Since the card (an Nvidia 8800GT variant) is still decently-powerful and has no obvious damage, I’m going to see what I can do under the limited-lifetime warranty. Compared to its earlier foibles, RMAing now ought to be unambiguous and straightforward, given that the computer now refuses to even POST if the card is present.