XPS 8940, updates installed, boot loop

My XPS 8940 was working just fine, until last night I accepted a prompt from Dell to install updates. I don't even remember what these updates were (when I went to bed, the screen said installing updates, 3% complete, don't turn off your computer, etc.).

This morning, the PC is stuck in a boot loop. The Dell logo pops up for less than 2 seconds (I timed it), then the PC shuts down, then it restarts, the Dell logo pops up for less than 2 seconds, the PC shuts down, etc.

I tried punching F2, F8, F12, the only thing that stops it is unplugging the power supply (but if I plug it back in, the cycle starts again instantly!).

This is a stock XPS 8940 with an upgraded 500W power supply.